Account validation email action and view

What does this MR do and why?

This MR is part of #346233 (closed).

It adds the email view and mailer action for Account Validation Email. This email will be sent (sending is not implemented in this MR) to a pipeline creator when their pipeline fails because their account is not validated. This failure happens to prevent abuse.

References

Design: https://gitlab.com/gitlab-org/growth/team-tasks/-/issues/463 Why are we sending this email?: #346233 (closed)

Screenshots or screen recordings

HTML	Text

How to set up and validate locally

1. Grab a pipeline from the database

   # In Rails console (run `bin/rails c` in `/path/to/gitlab-development-kit/gitlab`)
   > pipeline = Ci::Pipeline.last

2. Send the email

   # Still in Rails console
   > Notify.account_validation_email(pipeline, 'test@example.com').deliver_now

3. Open letter_opener and validate the content of the email

   It should look like this 👇🏼

   

MR acceptance checklist

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