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

spec/fabricators/announcement_fabricator.rb

@@ -0,0 +1,6 @@
+Fabricator(:announcement) do
+  text      { Faker::Lorem.paragraph(sentence_count: 2) }
+  published true
+  starts_at nil
+  ends_at   nil
+end

spec/fabricators/announcement_mute_fabricator.rb

@@ -0,0 +1,4 @@
+Fabricator(:announcement_mute) do
+  account
+  announcement
+end

spec/fabricators/announcement_reaction_fabricator.rb

@@ -0,0 +1,5 @@
+Fabricator(:announcement_reaction) do
+  account
+  announcement
+  name '🌿'
+end