Add ability to follow hashtags (#18809)

app/serializers/rest/tag_serializer.rb

@@ -5,6 +5,8 @@ class REST::TagSerializer < ActiveModel::Serializer
 
   attributes :name, :url, :history
 
+  attribute :following, if: :current_user?
+
   def url
     tag_url(object)
   end
@@ -12,4 +14,16 @@ class REST::TagSerializer < ActiveModel::Serializer
   def name
     object.display_name
   end
+
+  def following
+    if instance_options && instance_options[:relationships]
+      instance_options[:relationships].following_map[object.id] || false
+    else
+      TagFollow.where(tag_id: object.id, account_id: current_user.account_id).exists?
+    end
+  end
+
+  def current_user?
+    !current_user.nil?
+  end
 end