Add explanation to mute dialog, refactor and clean up mute/block UI (#11992)

* Add some explanation to the mute modal dialog

* Remove `isSubmitting` from mute modal code, this wasn't used

* Refactor block modal

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

* Refactor SCSS a bit

* Put mute modal toggle to the same side as in the report dialog for consistency

* Reword mute explanation

* Fix mute explanation styling

* Left-align all text in mute confirmation modal

app/javascript/mastodon/features/ui/util/async-components.js

@@ -106,6 +106,10 @@ export function MuteModal () {
   return import(/* webpackChunkName: "modals/mute_modal" */'../components/mute_modal');
 }
 
+export function BlockModal () {
+  return import(/* webpackChunkName: "modals/block_modal" */'../components/block_modal');
+}
+
 export function ReportModal () {
   return import(/* webpackChunkName: "modals/report_modal" */'../components/report_modal');
 }