Use common layout for merge request email templates

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The following discussion from !33460 (merged) should be addressed:

• @tkuah started a discussion:

  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
• ee/app/views/notify/approved_merge_request_email.html.haml
• ee/app/views/notify/unapproved_merge_request_email.html.haml

The following elements can be DRYed:

1. %html.head
2. %body