Include the reason for emailing in the new_mention emails

app/views/notify/new_mention_in_issue_email.html.haml

+%p
+  You have been mentioned in an issue.
+
 - if current_application_settings.email_author_in_body
   %div
     #{link_to @issue.author_name, user_url(@issue.author)} wrote:

app/views/notify/new_mention_in_merge_request_email.html.haml

+%p
+  You have been mentioned in Merge Request #{@merge_request.to_reference}
+
 - if current_application_settings.email_author_in_body
   %div
     #{link_to @merge_request.author_name, user_url(@merge_request.author)} wrote: