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

config/routes.rb

@@ -114,6 +114,7 @@ Rails.application.routes.draw do
   resources :tags,   only: [:show]
   resources :emojis, only: [:show]
   resources :invites, only: [:index, :create, :destroy]
+  resources :filters, except: [:show]
 
   get '/media_proxy/:id/(*any)', to: 'media_proxy#show', as: :media_proxy
 
@@ -254,6 +255,7 @@ Rails.application.routes.draw do
       resources :mutes,      only: [:index]
       resources :favourites, only: [:index]
       resources :reports,    only: [:index, :create]
+      resources :filters,    only: [:index, :create, :show, :update, :destroy]
 
       namespace :apps do
         get :verify_credentials, to: 'credentials#show'