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

db/migrate/20180628181026_create_custom_filters.rb

@@ -0,0 +1,13 @@
+class CreateCustomFilters < ActiveRecord::Migration[5.2]
+  def change
+    create_table :custom_filters do |t|
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }
+      t.datetime :expires_at
+      t.text :phrase, null: false, default: ''
+      t.string :context, array: true, null: false, default: []
+      t.boolean :irreversible, null: false, default: false
+
+      t.timestamps
+    end
+  end
+end