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.