WIP: Fix merge request approval layout

Open username-removed-636429 requested to merge fix-merge-request-approval-layout into master

Utilizes the newly-created mailer.html.haml template for the header and footer of merge request approve/unapprove email notifications since they are identical in this case. Also allows these notification emails to take advantage of the custom branding feature added in gitlab-ce!9049.

Fixes #1898

See also: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9049#note_23966476

I will open a MR in gitlab CE to backport the changes in 5db2f07a