* 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

Just don't try to save space by only selecting few attributes. If
anyone is wondering, this is needed because the emoji entity cache
is not really only used for entities, it's accessed again to
generate Emoji tags in ActivityPub/OStatus, so a lot more properties
are used than what is needed in HTML alone...

* Add entity cache

Use a caching layer for mentions and custom emojis that are
dynamically extracted from text.

Reduce duplicate text extractions

* Fix code style issue