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/mastodon/features/ui/components/media_modal.js

@@ -211,7 +211,6 @@ class MediaModal extends ImmutablePureComponent {
             style={swipeableViewsStyle}
             containerStyle={containerStyle}
             onChangeIndex={this.handleSwipe}
-            onSwitching={this.handleSwitching}
             index={index}
           >
             {content}