test+admin login issue

This is branched off the forgot_password because the helper for that is needed here too.

The idea behind this fix is easy: if someone logs in with CAS to Canvas but their user doesn't exist in Canvas, shoot them back to the main site. We know their user exists there, so it can give them a relevant screen for themselves.

Companion PR: https://github.com/beyond-z/beyondz-platform/pull/250
What is the advantage of doing this in code vs just reading a config value?  I feel like this should come from config.