Mailer: use custom header logo for merge request emails

What does this MR do and why?

Email notifications are sent for various events. At the top of the email, the logo is placed - either the custom header logo (instance setting in admin area), or otherwise the GitLab logo. Unfortunately, the custom header logo is not considered in some emails.

This MR ensures that the custom header logo is also used for the following emails:

• approved_merge_request_email
• unapproved_merge_request_email
• merge_when_pipeline_succeeds_email

🛠 with ❤ at Siemens

/cc @bufferoverflow

Screenshots

Email	Before MR	After MR (if custom header logo set)
approved_merge_request_email
unapproved_merge_request_email
merge_when_pipeline_succeeds_email

How to set up and validate locally

1. Navigate to Mailer Previews: http://localhost:3000/rails/mailers
2. View approved_merge_request_email preview: http://localhost:3000/rails/mailers/notify/approved_merge_request_email
3. View unapproved_merge_request_email preview: http://localhost:3000/rails/mailers/notify/unapproved_merge_request_email
4. View merge_when_pipeline_succeeds_email preview: http://localhost:3000/rails/mailers/notify/merge_when_pipeline_succeeds_email
5. Sign in as administrator and navigate to Admin Area > Appearance > Navigation: http://localhost:3000/admin/application_settings/appearance
6. Upload a custom Header logo.
7. Refresh the Mailer Preview pages.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.