Sends email notifications on attention requested requests

What does this MR do and why?

Sends an email notification to the assignee/reviewer whenever their attention is requested.

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:mr_attention_requests)

2. Visit http://localhost:3000/rails/letter_opener to view outgoing emails.
3. Visit any project and create an MR
4. Assign a user to the MR
5. Click the star icon next to the assignee name to create an attention request.
6. Observe a new email sent with the text {user} requested your attention on !{MR ID}.

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.

Closes #345366 (closed)