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/index.html.haml

@@ -0,0 +1,30 @@
+- content_for :page_title do
+  = t('admin.announcements.title')
+
+- content_for :heading_actions do
+  = link_to t('admin.announcements.new.title'), new_admin_announcement_path, class: 'button'
+
+.filters
+  .filter-subset
+    %strong= t('admin.relays.status')
+    %ul
+      %li= filter_link_to t('generic.all'), published: nil, unpublished: nil
+      %li= filter_link_to safe_join([t('admin.announcements.live'), "(#{number_with_delimiter(Announcement.published.count)})"], ' '), published: '1', unpublished: nil
+
+- if @announcements.empty?
+  %div.muted-hint.center-text
+    = t 'admin.announcements.empty'
+- else
+  .table-wrapper
+    %table.table
+      %thead
+        %tr
+          %th= t('simple_form.labels.announcement.text')
+          %th= t('admin.announcements.time_range')
+          %th= t('admin.announcements.published')
+          %th
+      %tbody
+        = render partial: 'announcement', collection: @announcements
+
+= paginate @announcements
+