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

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