Keyword/phrase filtering (#7905)

* Add keyword filtering

    GET|POST       /api/v1/filters
    GET|PUT|DELETE /api/v1/filters/:id

- Irreversible filters can drop toots from home or notifications
- Other filters can hide toots through the client app
- Filters use a phrase valid in particular contexts, expiration

* Make sure expired filters don't get applied client-side

* Add missing API methods

* Remove "regex filter" from column settings

* Add tests

* Add test for FeedManager

* Add CustomFilter test

* Add UI for managing filters

* Add streaming API event to allow syncing filters

* Fix tests

config/locales/en.yml

@@ -474,6 +474,22 @@ en:
     follows: You follow
     mutes: You mute
     storage: Media storage
+  filters:
+    contexts:
+      home: Home timeline
+      notifications: Notifications
+      public: Public timelines
+      thread: Conversations
+    edit:
+      title: Edit filter
+    errors:
+      invalid_context: None or invalid context supplied
+      invalid_irreversible: Irreversible filtering only works with home or notifications context
+    index:
+      delete: Delete
+      title: Filters
+    new:
+      title: Add new filter
   followers:
     domain: Domain
     explanation_html: If you want to ensure the privacy of your statuses, you must be aware of who is following you. <strong>Your private statuses are delivered to all instances where you have followers</strong>. You may wish to review them, and remove followers if you do not trust your privacy to be respected by the staff or software of those instances.