Change how unread announcements are handled (#13020)

* Change meaning of /api/v1/announcements/:id/dismiss to mark an announcement as read

* Change how unread announcements are counted in UI

* Add unread marker to announcements and mark announcements as unread as they are displayed

* Fixups

app/serializers/rest/announcement_serializer.rb

@@ -4,15 +4,25 @@ class REST::AnnouncementSerializer < ActiveModel::Serializer
   attributes :id, :content, :starts_at, :ends_at, :all_day,
              :published_at, :updated_at
 
+  attribute :read, if: :current_user?
+
   has_many :mentions
   has_many :tags, serializer: REST::StatusSerializer::TagSerializer
   has_many :emojis, serializer: REST::CustomEmojiSerializer
   has_many :reactions, serializer: REST::ReactionSerializer
 
+  def current_user?
+    !current_user.nil?
+  end
+
   def id
     object.id.to_s
   end
 
+  def read
+    object.announcement_mutes.where(account: current_user.account).exists?
+  end
+
   def content
     Formatter.instance.linkify(object.text)
   end