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/views/admin/announcements/edit.html.haml

@@ -0,0 +1,22 @@
+- content_for :page_title do
+  = t('.title')
+
+= simple_form_for @announcement, url: admin_announcement_path(@announcement) do |f|
+  = render 'shared/error_messages', object: @announcement
+
+  .fields-group
+    = f.input :starts_at, include_blank: true, wrapper: :with_block_label
+    = f.input :ends_at, include_blank: true, wrapper: :with_block_label
+
+  .fields-group
+    = f.input :all_day, as: :boolean, wrapper: :with_label
+
+  .fields-group
+    = f.input :text, wrapper: :with_block_label
+
+  - if @announcement.scheduled_at.present? && !@announcement.published?
+    .fields-group
+      = f.input :scheduled_at, include_blank: true, wrapper: :with_block_label
+
+  .actions
+    = f.button :button, t('generic.save_changes'), type: :submit