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/models/custom_emoji.rb

@@ -67,7 +67,7 @@ class CustomEmoji < ApplicationRecord
   end
 
   class << self
-    def from_text(text, domain)
+    def from_text(text, domain = nil)
       return [] if text.blank?
 
       shortcodes = text.scan(SCAN_RE).map(&:first).uniq