User admin approval - Email user when approved

What does this MR do?

Email the user when their request to join an instance is approved by the instance admin.

Sanity testing:

• Enable 'Require admin approval for new sign-ups' in Admin > Settings > General > Sign-up restrictions
• Log out of root
• Click 'Register now' and sign up with a new fake user. You'll see the notice "You have signed up successfully. However, we could not sign you in because your account is awaiting approval from your GitLab administrator"
• Log back into root
• Make sure rails-background-jobs and rails-web aren't stale-- could be good to do a gdk restart rails here just to be safe
• Admin > Users > Pending approval > Approve the user
• You'll see the job enqueued at http://127.0.0.1:3000/admin/sidekiq/queues/mailers, and after a few seconds it'll show up in http://localhost:3000/rails/letter_opener/
• Check that the html and plaintext emails have the correct information

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #257880 (closed)