Refactor features.py to allow overriding other functions
Refactor RegistrationView to handle extra fields and instance invites

Leans more heavily on default implementation of Django Registration

Adds in support for new user fields

Adds an additional overridable function, send_user_activation, to determine
whether or not to send an activation email
Fix password reset forms

If Django Admin templates are higher in the template loading order,
Django registration will try to use the Admin site's password reset
templates
Redirect to the map page on activation when signing up from an instance

Also refactors ActivationView to only override functionality we actually
want to change