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/features/ui/util/async-components.js

@@ -90,6 +90,10 @@ export function Blocks () {
   return import(/* webpackChunkName: "features/blocks" */'../../blocks');
 }
 
+export function DomainBlocks () {
+  return import(/* webpackChunkName: "features/domain_blocks" */'../../domain_blocks');
+}
+
 export function Mutes () {
   return import(/* webpackChunkName: "features/mutes" */'../../mutes');
 }