Email notifications for remote mirror errors uses wrong layout

The "Remote mirror update failed" email notification (added in gitlab-ce!23595) appears to be using the HTML email template designed for our CI failures, but it does not include the full layout wrapper designed for it. As a result, it looks a bit awkward with some text the same color as the background, icons floating off-center, inconsistent margins, etc.

remote mirror notification (how it currently looks) standard CI notification (how it should look)
Screen_Shot_2019-07-15_at_6.06.57_PM Screen_Shot_2019-07-15_at_6.07.18_PM

It appears the HTML template was copy/pasted for this use, but never actually tested.