Use common layout for merge request email templates
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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`
issue