* Change order of announcements in admin page to sort show newly-created first

Fixes #15090

* Use reverse-chronological rather than creation date only

* Fix PostgreSQL load when linking in announcements

Fixes #13245 by caching status lookups

Since statuses are supposed to be known already and we only
need their URLs and a few other things, caching them should
be fine.

Since it's only used by announcements so far, there won't
be much statuses to cache.

* Perform status lookup when saving announcements, not when rendering them

* Change EntityCache#status to fetch URLs instead of looking into the database

* Move announcement link lookup to publishing worker

* Address issues pointed out during review

* Add support for links to public statuses in announcements to be opened in WebUI

* Please CodeClimate

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

* 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