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

app/models/account.rb

@@ -99,6 +99,7 @@ class Account < ApplicationRecord
   has_many :targeted_reports, class_name: 'Report', foreign_key: :target_account_id
 
   has_many :report_notes, dependent: :destroy
+  has_many :custom_filters, inverse_of: :account, dependent: :destroy
 
   # Moderation notes
   has_many :account_moderation_notes, dependent: :destroy