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/account.rb

@@ -476,6 +476,12 @@ class Account < ApplicationRecord
       records
     end
 
+    def from_text(text)
+      return [] if text.blank?
+
+      text.scan(MENTION_RE).map { |match| match.first.split('@', 2) }.uniq.map { |(username, domain)| EntityCache.instance.mention(username, domain) }
+    end
+
     private
 
     def generate_query_for_search(terms)