Use common layout for merge request email templates
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=229150) </details> <!--IssueSummary end--> The following discussion from !33460 should be addressed: - [ ] @tkuah started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33460#note_379744241): > I diffed with the approved template with `tkdiff ee/app/views/notify/approved_merge_request_email.html.haml app/views/notify/merge_when_pipeline_succeeds_email.html.haml`. Looks similar to that template except for externationalization and removal as slashes, as expected. > > It will be nice to use layouts to DRY this up but this can be done as a follow-up. Several mail templates could be DRYed to use common layout - [app/views/notify/merge_when_pipeline_succeeds_email.html.haml](app/views/notify/merge_when_pipeline_succeeds_email.html.haml) - [ee/app/views/notify/approved_merge_request_email.html.haml](ee/app/views/notify/approved_merge_request_email.html.haml) - [ee/app/views/notify/unapproved_merge_request_email.html.haml](ee/app/views/notify/unapproved_merge_request_email.html.haml) The following elements can be DRYed: 1. `%html.head` 1. `%body`
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