Migrate glitch-soc keyword mutes to Mastodon's

Completely remove glitch-soc's Keyword Mutes, migrate
existing database records to CustomFilters.

Handling of client-side filters is still not implemented
in the glitch-soc front-end.

spec/controllers/settings/keyword_mutes_controller_spec.rb

@@ -1,5 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe Settings::KeywordMutesController, type: :controller do
-
-end