Add some UI for user-defined domain blocks (#6628)

* Keep list of blocked domains

Might be overkill, but I'm trying to follow the same logic as for blocked users

* Add basic domain block UI

* Add the domain blocks UI to Getting Started

* Fix undefined URL in `fetchDomainBlocks`

* Update all known users' domain_blocking relationship instead of just one's

app/javascript/mastodon/reducers/index.js

@@ -6,6 +6,7 @@ import alerts from './alerts';
 import { loadingBarReducer } from 'react-redux-loading-bar';
 import modal from './modal';
 import user_lists from './user_lists';
+import domain_lists from './domain_lists';
 import accounts from './accounts';
 import accounts_counters from './accounts_counters';
 import statuses from './statuses';
@@ -34,6 +35,7 @@ const reducers = {
   loadingBar: loadingBarReducer,
   modal,
   user_lists,
+  domain_lists,
   status_lists,
   accounts,
   accounts_counters,