Add option to disable email, password and name changing

What does this MR do?

These options gives the system admin the ability to disable a user from changing their primary email, their given name at registration, and their password.

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

It's worth going over all of it -- this is my first MR.

Why was this MR needed?

This is useful for configurations which primarily integrate with omniauth services as it prevents double-account creation (due to changing their email) and the restriction of only using the authentication service instead of utilising GitLabs authenticataion mechanismsm, hence disabling changing of password. In addition, it gives the sysadmin the ability to allow the authentication service to provide the email and name attributes only.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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