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/styles/mastodon-light/diff.scss

@@ -310,6 +310,7 @@ html {
 .boost-modal,
 .confirmation-modal,
 .mute-modal,
+.block-modal,
 .report-modal,
 .embed-modal,
 .error-modal,
@@ -326,6 +327,7 @@ html {
 .boost-modal__action-bar,
 .confirmation-modal__action-bar,
 .mute-modal__action-bar,
+.block-modal__action-bar,
 .onboarding-modal__paginator,
 .error-modal__footer {
   background: darken($ui-base-color, 6%);