Add support for editing media description and focus point of already-posted statuses (#20878)

* Add backend support for editing media attachments of existing posts

* Allow editing media attachments of already-posted toots

* Add tests

app/javascript/mastodon/reducers/compose.js

@@ -444,7 +444,7 @@ export default function compose(state = initialState, action) {
       .setIn(['media_modal', 'dirty'], false)
       .update('media_attachments', list => list.map(item => {
         if (item.get('id') === action.media.id) {
-          return fromJS(action.media).set('unattached', true);
+          return fromJS(action.media).set('unattached', !action.attached);
         }
 
         return item;