LDAP username case-insensivity

LDAP username case-insensivity

This patches a case sensitivity issue in the core.

LDAP itself is case insensitive, but the LDAP module of the core will
keep the case of the usename when creating a local Quassel account.
Thus, if you login with the same username but using a different casing,
a new internal Quassel account will be creating, resulting in an
apparent settings loss (networks, ...).

The patch converts the username to lowercase before handing it to the
core.