feat: Add message content to mention email template (#2838)

app/drops/message_drop.rb

@@ -0,0 +1,12 @@
+class MessageDrop < BaseDrop
+  include MessageFormatHelper
+
+  def sender_display_name
+    @obj.sender.try(:display_name)
+  end
+
+  def text_content
+    content = @obj.try(:content)
+    transform_user_mention_content content
+  end
+end