Add announcements (#12662)

* Add announcements

Fix #11006

* Add reactions to announcements

* Add admin UI for announcements

* Add unit tests

* Fix issues

- Add `with_dismissed` param to announcements API
- Fix end date not being formatted when time range is given
- Fix announcement delete causing reactions to send streaming updates
- Fix announcements container growing too wide and mascot too small
- Fix `all_day` being settable when no time range is given
- Change text "Update" to "Announcement"

* Fix scheduler unpublishing announcements before they are due

* Fix filter params not being passed to announcements filter

app/javascript/styles/mastodon/forms.scss

@@ -222,6 +222,12 @@ code {
     }
   }
 
+  .input.datetime .label_input select {
+    display: inline-block;
+    width: auto;
+    flex: 0;
+  }
+
   .required abbr {
     text-decoration: none;
     color: lighten($error-value-color, 12%);