[Glitch] Add explanation to mute dialog, refactor and clean up mute/block UI

Port 9027bfff0c to glitch-soc

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

app/javascript/flavours/glitch/styles/mastodon-light/diff.scss

@@ -226,6 +226,7 @@
 .boost-modal,
 .confirmation-modal,
 .mute-modal,
+.block-modal,
 .report-modal,
 .embed-modal,
 .error-modal,
@@ -236,6 +237,7 @@
 .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%);