Added test to verify that users can verify their accounts with the verify_user email link in subdomains with client redirects.

[#82406674]
Fixed bug where creating an account without a client_id wouldn't
redirect correctly afterwards.

Clients with empty ID strings are handled correctly when generating
activation emails.

Creating accouts in subdomains with and without clients now works.

Changed activation email to show identity zone name (if there is one).

[Finishes #82406674, Fixes #83898390]