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.

config/routes.rb

@@ -77,12 +77,6 @@ Rails.application.routes.draw do
   namespace :settings do
     resource :profile, only: [:show, :update]
 
-    resources :keyword_mutes do
-      collection do
-        delete :destroy_all
-      end
-    end
-
     resource :preferences, only: [:show, :update]
     resource :notifications, only: [:show, :update]
     resource :import, only: [:show, :create]