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.

config/locales/en.yml

@@ -1124,15 +1124,24 @@ en:
       public: Public timelines
       thread: Conversations
     edit:
+      add_keyword: Add keyword
+      keywords: Keywords
       title: Edit filter
     errors:
+      deprecated_api_multiple_keywords: These parameters cannot be changed from this application because they apply to more than one filter keyword. Use a more recent application or the web interface.
       invalid_context: None or invalid context supplied
-      invalid_irreversible: Irreversible filtering only works with home or notifications context
     index:
+      contexts: Filters in %{contexts}
       delete: Delete
       empty: You have no filters.
+      expires_in: Expires in %{distance}
+      expires_on: Expires on %{date}
+      keywords:
+        one: "%{count} keyword"
+        other: "%{count} keywords"
       title: Filters
     new:
+      save: Save new filter
       title: Add new filter
   footer:
     developers: Developers

config/locales/simple_form.en.yml

@@ -68,6 +68,11 @@ en:
         with_dns_records: An attempt to resolve the given domain's DNS records will be made and the results will also be blocked
       featured_tag:
         name: 'You might want to use one of these:'
+      filters:
+        action: Chose which action to perform when a post matches the filter
+        actions:
+          hide: Completely hide the filtered content, behaving as if it did not exist
+          warn: Hide the filtered content behind a warning mentioning the filter's title
       form_challenge:
         current_password: You are entering a secure area
       imports:
@@ -181,6 +186,7 @@ en:
         setting_use_pending_items: Slow mode
         severity: Severity
         sign_in_token_attempt: Security code
+        title: Title
         type: Import type
         username: Username
         username_or_email: Username or Email
@@ -189,6 +195,10 @@ en:
         with_dns_records: Include MX records and IPs of the domain
       featured_tag:
         name: Hashtag
+      filters:
+        actions:
+          hide: Hide completely
+          warn: Hide with a warning
       interactions:
         must_be_follower: Block notifications from non-followers
         must_be_following: Block notifications from people you don't follow