Serialize mentions in the order they are added (#6836)

Up until now, the order seemed to be in the *opposite* order,
which caused the WebUI to populate mentions in reversed order
when replying to toots local to one's instance.

app/serializers/rest/status_serializer.rb

@@ -15,7 +15,7 @@ class REST::StatusSerializer < ActiveModel::Serializer
   belongs_to :account, serializer: REST::AccountSerializer
 
   has_many :media_attachments, serializer: REST::MediaAttachmentSerializer
-  has_many :mentions
+  has_many :ordered_mentions, key: :mentions
   has_many :tags
   has_many :emojis, serializer: REST::CustomEmojiSerializer
 
@@ -86,6 +86,10 @@ class REST::StatusSerializer < ActiveModel::Serializer
       %w(public unlisted).include?(object.visibility)
   end
 
+  def ordered_mentions
+    object.mentions.order(:id)
+  end
+
   class ApplicationSerializer < ActiveModel::Serializer
     attributes :name, :website
   end