Force notifications to SAML email address for dedicated users [UX]

UX issue for https://gitlab.com/gitlab-org/gitlab-ee/issues/5332. Blocking https://gitlab.com/gitlab-org/gitlab-ee/issues/9372

Overview

If dedicated credentials are enabled:

• On creating the new user, force notifications for members to the e-mail address we get back in the SAML response. We'll need to consider how this changes https://gitlab.com/gitlab-org/gitlab-ee/issues/9374 in this case (the new user's email should be read-only).
• In profile/notifications, do not allow the user to change their notification email away from this e-mail address. We could handle this with an error, but a better approach may be showing some indication in the UI (and why).
• In profile/emails, do not allow the user to change their primary, commit, or notification email.

Solution

On creating a new user, force notifications for members to the email address we get back in the SAML repsonse:

Do not allow the user to change their notification email from this e-mail address: