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

config/navigation.rb

@@ -46,6 +46,7 @@ SimpleNavigation::Configuration.run do |navigation|
     n.item :admin, safe_join([fa_icon('cogs fw'), t('admin.title')]), admin_dashboard_url, if: proc { current_user.staff? } do |s|
       s.item :dashboard, safe_join([fa_icon('tachometer fw'), t('admin.dashboard.title')]), admin_dashboard_url
       s.item :settings, safe_join([fa_icon('cogs fw'), t('admin.settings.title')]), edit_admin_settings_url, if: -> { current_user.admin? }, highlights_on: %r{/admin/settings}
+      s.item :announcements, safe_join([fa_icon('bullhorn fw'), t('admin.announcements.title')]), admin_announcements_path, highlights_on: %r{/admin/announcements}
       s.item :custom_emojis, safe_join([fa_icon('smile-o fw'), t('admin.custom_emojis.title')]), admin_custom_emojis_url, highlights_on: %r{/admin/custom_emojis}
       s.item :relays, safe_join([fa_icon('exchange fw'), t('admin.relays.title')]), admin_relays_url, if: -> { current_user.admin? && !whitelist_mode? }, highlights_on: %r{/admin/relays}
       s.item :sidekiq, safe_join([fa_icon('diamond fw'), 'Sidekiq']), sidekiq_url, link_html: { target: 'sidekiq' }, if: -> { current_user.admin? }