Add email notifications for RewriteHistory worker

What does this MR do and why?

Contributes to #483085 (closed)

Related RewriteWorker MR: !162306 (merged)

Problem

Synchrounous rewriting of repository history is too slow. We are going to make it asynchronous instead. But it's currently missing a notification channel for users to receive updates about the processing status.

Solution

Define email templates that is going to be called from RewriteHistory Sidekiq job.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots

Success	Failure

How to set up and validate locally

Check preview of both templates:

• http://localhost:3000/rails/mailers/notify/repository_rewrite_history_failure_email
• http://localhost:3000/rails/mailers/notify/repository_rewrite_history_success_email