Improve payload format of Web Push API now that it's open (#7521)

> Good lord what is happening in there

Previously the contents of the Web Push API payloads closely resembled the structure of JavaScript's [Notification](https://developer.mozilla.org/en-US/docs/Web/API/Notification). But now that the API is open to non-browser apps, and given that there is no required coupling between contents of the payload and a Notification object, here is how I changed the payload:

```json
{ 
  "access_token": "...",
  "preferred_locale": "en",
  "notification_id": "12345",
  "notification_type": "follow",
  "title": "So and so followed you",
  "body": "This is my bio",
  "icon": "https://example.com/avatar.png"
}
```

The title, body and icon attributes are included as a fallback so you can construct a minimal notification if you cannot perform a network request to the API to get more data.

app/serializers/web/notification_serializer.rb

@@ -2,168 +2,37 @@
 
 class Web::NotificationSerializer < ActiveModel::Serializer
   include RoutingHelper
-  include StreamEntriesHelper
+  include ActionView::Helpers::TextHelper
+  include ActionView::Helpers::SanitizeHelper
 
-  class DataSerializer < ActiveModel::Serializer
-    include RoutingHelper
-    include StreamEntriesHelper
-    include ActionView::Helpers::SanitizeHelper
+  attributes :access_token, :preferred_locale, :notification_id,
+             :notification_type, :icon, :title, :body
 
-    attributes :content, :nsfw, :url, :actions,
-               :access_token, :message, :dir
-
-    def content
-      decoder.decode(strip_tags(body))
-    end
-
-    def dir
-      rtl?(body) ? 'rtl' : 'ltr'
-    end
-
-    def nsfw
-      return if object.target_status.nil?
-      object.target_status.spoiler_text.presence
-    end
-
-    def url
-      case object.type
-      when :mention
-        web_url("statuses/#{object.target_status.id}")
-      when :follow
-        web_url("accounts/#{object.from_account.id}")
-      when :favourite
-        web_url("statuses/#{object.target_status.id}")
-      when :reblog
-        web_url("statuses/#{object.target_status.id}")
-      end
-    end
-
-    def actions
-      return @actions if defined?(@actions)
-
-      @actions = []
-
-      if object.type == :mention
-        @actions << expand_action if collapsed?
-        @actions << favourite_action
-        @actions << reblog_action if rebloggable?
-      end
-
-      @actions
-    end
-
-    def access_token
-      return if actions.empty?
-      current_push_subscription.associated_access_token
-    end
-
-    def message
-      I18n.t('push_notifications.group.title')
-    end
-
-    private
-
-    def body
-      case object.type
-      when :mention
-        object.target_status.text
-      when :follow
-        object.from_account.note
-      when :favourite
-        object.target_status.text
-      when :reblog
-        object.target_status.text
-      end
-    end
-
-    def decoder
-      @decoder ||= HTMLEntities.new
-    end
-
-    def expand_action
-      {
-        title: I18n.t('push_notifications.mention.action_expand'),
-        icon: full_asset_url('web-push-icon_expand.png', skip_pipeline: true),
-        todo: 'expand',
-        action: 'expand',
-      }
-    end
-
-    def favourite_action
-      {
-        title: I18n.t('push_notifications.mention.action_favourite'),
-        icon: full_asset_url('web-push-icon_favourite.png', skip_pipeline: true),
-        todo: 'request',
-        method: 'POST',
-        action: "/api/v1/statuses/#{object.target_status.id}/favourite",
-      }
-    end
-
-    def reblog_action
-      {
-        title: I18n.t('push_notifications.mention.action_boost'),
-        icon: full_asset_url('web-push-icon_reblog.png', skip_pipeline: true),
-        todo: 'request',
-        method: 'POST',
-        action: "/api/v1/statuses/#{object.target_status.id}/reblog",
-      }
-    end
-
-    def collapsed?
-      !object.target_status.nil? && (object.target_status.sensitive? || object.target_status.spoiler_text.present?)
-    end
-
-    def rebloggable?
-      !object.target_status.nil? && !object.target_status.hidden?
-    end
+  def access_token
+    current_push_subscription.associated_access_token
   end
 
-  attributes :title, :image, :badge, :tag,
-             :timestamp, :icon
-
-  has_one :data, serializer: DataSerializer
-
-  def title
-    case object.type
-    when :mention
-      I18n.t('push_notifications.mention.title', name: name)
-    when :follow
-      I18n.t('push_notifications.follow.title', name: name)
-    when :favourite
-      I18n.t('push_notifications.favourite.title', name: name)
-    when :reblog
-      I18n.t('push_notifications.reblog.title', name: name)
-    end
+  def preferred_locale
+    current_push_subscription.associated_user&.locale || I18n.default_locale
   end
 
-  def image
-    return if object.target_status.nil? || object.target_status.media_attachments.empty?
-    full_asset_url(object.target_status.media_attachments.first.file.url(:small))
-  end
-
-  def badge
-    full_asset_url('badge.png', skip_pipeline: true)
-  end
-
-  def tag
+  def notification_id
     object.id
   end
 
-  def timestamp
-    object.created_at
+  def notification_type
+    object.type
   end
 
   def icon
-    object.from_account.avatar_static_url
+    full_asset_url(object.from_account.avatar_static_url)
   end
 
-  def data
-    object
+  def title
+    I18n.t("notification_mailer.#{object.type}.subject", name: object.from_account.display_name.presence || object.from_account.username)
   end
 
-  private
-
-  def name
-    display_name(object.from_account)
+  def body
+    truncate(strip_tags(object.target_status&.spoiler_text&.presence || object.target_status&.text || object.from_account.note), length: 140)
   end
 end