Change `all_day` to be a visual client-side cue only

Publish immediately if `scheduled_at` is in the past

Add `published_at` and `updated_at` to announcements JSON

* Fix spurious error and incorrect state change when adding a reaction twice

* Remove superfluous top border for announcements box

* Move announcements above scroll container; add button to temporarily hide them

* Remove interface for dismissing announcements

* Display number of unread announcements

* Count unread announcements accurately

* Fix size of announcement box not fitting the currently displayed announcement

* Fix announcement box background color to match button color

* 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