Core: in LDAP authenticator, don't try database auth with blank password

In the LDAP authenticator, we were trying to do database auth against
the core with a blank password (QString()), after LDAP auth has succeeded.
This was done because there was not another way to retrieve a quassel
UserId object for a given string username.

However, if we want to support migrating a user from Database to LDAP
auth, this causes problems-- we'd need to set the password column to
whatever an empty QString() maps to in the hashing algorithm.

It seems much simpler to just add a new method to core.h to look up
a UserId object in the current storage provider when we pass it
a string username. Then we can just call that method in the LDAP
authenticator.