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/concerns/expireable.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module Expireable
+  extend ActiveSupport::Concern
+
+  included do
+    scope :expired, -> { where.not(expires_at: nil).where('expires_at < ?', Time.now.utc) }
+
+    attr_reader :expires_in
+
+    def expires_in=(interval)
+      self.expires_at = interval.to_i.seconds.from_now unless interval.blank?
+      @expires_in     = interval
+    end
+
+    def expire!
+      touch(:expires_at)
+    end
+
+    def expired?
+      !expires_at.nil? && expires_at < Time.now.utc
+    end
+  end
+end