Attach existing contacts to new users created outside registration

Parallel issue: Contacts don't exist when a user registration email is sent, meaning tokens aren't available yet. Attached patch fixes both these issues with a refactor to how we handle registration.

This probably needs some more work to line up all the settings. We currently have a setting "Link to existing contacts" that essentially does nothing. This patch acts as if it is always turned on when user/contact connection is active -- so the setting is still useless.

However, this patch is necessary for the basic version of "create contact on registration" now, so we can't simply base this behavior on the Link to Existing setting.

This feels more like an entirely new behavior, and a new checkbox would have better backwards compatibility too, in case people are somehow counting on there not being a connection between users and redhen contacts when the user is created programmatically.

Maybe a new setting "Connect RedHen contacts to new users": "Will attempt to connect Drupal users to RedHen contacts by matching email addresses whenever a user is created."

And, as you've noted, the "registration_link" aka "Link to existing contacts" is not used at all, so it could be taken off the settings form in the "Registration" section, and the new checkbox could replace it (but not be in the "Registration" field group).

Thanks for the feedback @mariacha1. How about now? I went ahead and added a schema file for the Contact settings as well.