Revamp post filtering system (#18058)

* Add model for custom filter keywords

* Use CustomFilterKeyword internally

Does not change the API

* Fix /filters/edit and /filters/new

* Add migration tests

* Remove whole_word column from custom_filters (covered by custom_filter_keywords)

* Redesign /filters

Instead of a list, present a card that displays more information and handles
multiple keywords per filter.

* Redesign /filters/new and /filters/edit to add and remove keywords

This adds a new gem dependency: cocoon, as well as a npm dependency:
cocoon-js-vanilla. Those are used to easily populate and remove form fields
from the user interface when manipulating multiple keyword filters at once.

* Add /api/v2/filters to edit filter with multiple keywords

Entities:
- `Filter`: `id`, `title`, `filter_action` (either `hide` or `warn`), `context`
  `keywords`
- `FilterKeyword`: `id`, `keyword`, `whole_word`

API endpoits:
- `GET /api/v2/filters` to list filters (including keywords)
- `POST /api/v2/filters` to create a new filter
  `keywords_attributes` can also be passed to create keywords in one request
- `GET /api/v2/filters/:id` to read a particular filter
- `PUT /api/v2/filters/:id` to update a new filter
  `keywords_attributes` can also be passed to edit, delete or add keywords in
   one request
- `DELETE /api/v2/filters/:id` to delete a particular filter
- `GET /api/v2/filters/:id/keywords` to list keywords for a filter
- `POST /api/v2/filters/:filter_id/keywords/:id` to add a new keyword to a
   filter
- `GET /api/v2/filter_keywords/:id` to read a particular keyword
- `PUT /api/v2/filter_keywords/:id` to edit a particular keyword
- `DELETE /api/v2/filter_keywords/:id` to delete a particular keyword

* Change from `irreversible` boolean to `action` enum

* Remove irrelevent `irreversible_must_be_within_context` check

* Fix /filters/new and /filters/edit with update for filter_action

* Fix Rubocop/Codeclimate complaining about task names

* Refactor FeedManager#phrase_filtered?

This moves regexp building and filter caching to the `CustomFilter` class.

This does not change the functional behavior yet, but this changes how the
cache is built, doing per-custom_filter regexps so that filters can be matched
independently, while still offering caching.

* Perform server-side filtering and output result in REST API

* Fix numerous filters_changed events being sent when editing multiple keywords at once

* Add some tests

* Use the new API in the WebUI

- use client-side logic for filters we have fetched rules for.
  This is so that filter changes can be retroactively applied without
  reloading the UI.
- use server-side logic for filters we haven't fetched rules for yet
  (e.g. network error, or initial timeline loading)

* Minor optimizations and refactoring

* Perform server-side filtering on the streaming server

* Change the wording of filter action labels

* Fix issues pointed out by linter

* Change design of “Show anyway” link in accordence to review comments

* Drop “irreversible” filtering behavior

* Move /api/v2/filter_keywords to /api/v1/filters/keywords

* Rename `filter_results` attribute to `filtered`

* Rename REST::LegacyFilterSerializer to REST::V1::FilterSerializer

* Fix systemChannelId value in streaming server

* Simplify code by removing client-side filtering code

The simplifcation comes at a cost though: filters aren't retroactively
applied anymore.

app/lib/feed_manager.rb

@@ -352,7 +352,6 @@ class FeedManager
   def filter_from_home?(status, receiver_id, crutches)
     return false if receiver_id == status.account_id
     return true  if status.reply? && (status.in_reply_to_id.nil? || status.in_reply_to_account_id.nil?)
-    return true  if phrase_filtered?(status, receiver_id, :home)
 
     check_for_blocks = crutches[:active_mentions][status.id] || []
     check_for_blocks.concat([status.account_id])
@@ -388,7 +387,6 @@ class FeedManager
   # @return [Boolean]
   def filter_from_mentions?(status, receiver_id)
     return true if receiver_id == status.account_id
-    return true if phrase_filtered?(status, receiver_id, :notifications)
 
     # This filter is called from NotifyService, but already after the sender of
     # the notification has been checked for mute/block. Therefore, it's not
@@ -418,34 +416,6 @@ class FeedManager
     false
   end
 
-  # Check if the status hits a phrase filter
-  # @param [Status] status
-  # @param [Integer] receiver_id
-  # @param [Symbol] context
-  # @return [Boolean]
-  def phrase_filtered?(status, receiver_id, context)
-    active_filters = Rails.cache.fetch("filters:#{receiver_id}") { CustomFilter.where(account_id: receiver_id).active_irreversible.to_a }.to_a
-
-    active_filters.select! { |filter| filter.context.include?(context.to_s) && !filter.expired? }
-
-    active_filters.map! do |filter|
-      if filter.whole_word
-        sb = /\A[[:word:]]/.match?(filter.phrase) ? '\b' : ''
-        eb = /[[:word:]]\z/.match?(filter.phrase) ? '\b' : ''
-
-        /(?mix:#{sb}#{Regexp.escape(filter.phrase)}#{eb})/
-      else
-        /#{Regexp.escape(filter.phrase)}/i
-      end
-    end
-
-    return false if active_filters.empty?
-
-    combined_regex = Regexp.union(active_filters)
-
-    combined_regex.match?(status.proper.searchable_text)
-  end
-
   # Adds a status to an account's feed, returning true if a status was
   # added, and false if it was not added to the feed. Note that this is
   # an internal helper: callers must call trim or push updates if