Allow admins to stop impersonating users without e-mail addresses

What does this MR do?

Allows admins to stop impersonating users who are missing e-mail addresses, e.g. those who signed up using OAuth.

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

I added tests to spec/controller/admin/impersonations_controller_spec.rb, and moved the tests for success to a shared example. I'm not sure this was the best approach or if there should be tests added elsewhere instead or in addition to those added.

Why was this MR needed?

See #24576 (closed).

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?

#24576 (closed)