Focal points (#6520)

* Add focus param to media API, center thumbnails on focus point

* Add UI for setting a focal point

* Improve focal point icon on upload item

* Use focal point in upload preview

* Add focalPoint property to ActivityPub

* Don't show focal point button for non-image attachments

app/javascript/mastodon/reducers/compose.js

@@ -265,7 +265,7 @@ export default function compose(state = initialState, action) {
       .set('is_submitting', false)
       .update('media_attachments', list => list.map(item => {
         if (item.get('id') === action.media.id) {
-          return item.set('description', action.media.description);
+          return fromJS(action.media);
         }
 
         return item;