Sync email address from specified omniauth provider

What does this MR do?

Provides option to sync a user's email address from omniauth provider email. When configured, the user's primary email address is not editable and is synced on sign-in. We intend it for use with auto_sign_in_with_provider and allow_single_sign_on configured to the same provider.

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

I have tested these code changes with cas3 and github omniauth. I'm not sure if there are concerns with other auth strategies.

Why was this MR needed?

We have our GitLab instance configured for single sign-on with another application. We want to ensure our users' email addresses in GitLab always match their email address in the other application.

Screenshots (if relevant)

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

What are the relevant issue numbers?