Add announcements (#12662)

* Add announcements

Fix #11006

* Add reactions to announcements

* Add admin UI for announcements

* Add unit tests

* Fix issues

- Add `with_dismissed` param to announcements API
- Fix end date not being formatted when time range is given
- Fix announcement delete causing reactions to send streaming updates
- Fix announcements container growing too wide and mascot too small
- Fix `all_day` being settable when no time range is given
- Change text "Update" to "Announcement"

* Fix scheduler unpublishing announcements before they are due

* Fix filter params not being passed to announcements filter

app/validators/reaction_validator.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class ReactionValidator < ActiveModel::Validator
+  SUPPORTED_EMOJIS = Oj.load(File.read(Rails.root.join('app', 'javascript', 'mastodon', 'features', 'emoji', 'emoji_map.json'))).keys.freeze
+
+  def validate(reaction)
+    return if reaction.name.blank? || reaction.custom_emoji_id.present?
+
+    reaction.errors.add(:name, I18n.t('reactions.errors.unrecognized_emoji')) unless unicode_emoji?(reaction.name)
+  end
+
+  private
+
+  def unicode_emoji?(name)
+    SUPPORTED_EMOJIS.include?(name)
+  end
+end