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/reducers/mutes.js

@@ -7,7 +7,6 @@ import {
 
 const initialState = Immutable.Map({
   new: Immutable.Map({
-    isSubmitting: false,
     account: null,
     notifications: true,
   }),
@@ -17,7 +16,6 @@ export default function mutes(state = initialState, action) {
   switch (action.type) {
   case MUTES_INIT_MODAL:
     return state.withMutations((state) => {
-      state.setIn(['new', 'isSubmitting'], false);
       state.setIn(['new', 'account'], action.account);
       state.setIn(['new', 'notifications'], true);
     });