create_preregistration_user is a footgun, because it takes the realm
from the request. The calling code is supposed to validate that
registration for the realm is allowed
first, but can sometimes do that on "realm" taken from something else
than the request - and later on calls create_preregistration_user, thus
leading to prereg user creation on unvalidated request.realm.
It's safer, and makes more sense, for this function to take the intended
realm as argument, instead of taking the entire request. It follows that
the same should be done for prepare_activation_url.
0c227217b2