Change how unread announcements are handled (#13020)

* Change meaning of /api/v1/announcements/:id/dismiss to mark an announcement as read

* Change how unread announcements are counted in UI

* Add unread marker to announcements and mark announcements as unread as they are displayed

* Fixups

app/javascript/mastodon/features/home_timeline/index.js

@@ -24,7 +24,7 @@ const mapStateToProps = state => ({
   hasUnread: state.getIn(['timelines', 'home', 'unread']) > 0,
   isPartial: state.getIn(['timelines', 'home', 'isPartial']),
   hasAnnouncements: !state.getIn(['announcements', 'items']).isEmpty(),
-  unreadAnnouncements: state.getIn(['announcements', 'unread']).size,
+  unreadAnnouncements: state.getIn(['announcements', 'items']).count(item => !item.get('read')),
   showAnnouncements: state.getIn(['announcements', 'show']),
 });