Increase fidelity of MR merged emails
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## Overview
* Emailed received by users when an MR is approved, or set to auto-merge are high-fidelity HTML emails.
* Conversely, emails received by users when an MR is merged are HTML although missing most of the design elements.

* We should update the MR merged email to match the design of other emails relating to MR status changes.
## Implementation Plan
* Update `app/views/notify/merged_merge_request_email.html.haml` to match the same template used in `app/views/notify/merge_when_pipeline_succeeds_email.html.haml` or other similar files.
issue