Only show one error message for an invalid email

What does this MR do?

Changes it to only validate the notification_email format if it's different from email.

Are there points in the code the reviewer needs to double check?

Just the one place I changed the User model.

Why was this MR needed?

If an admin created a user with an invalid email, it would display two errors. One for the email attribute, and one for the notification_email that is set from it.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #5905 (closed)