ApiBlock: Improve username validation

ApiBlock: Improve username validation

The current username validation lets any invalid username through, on
the assumption that it's an IP address. We can do better: call the
backend to get the actual type and target, and reject anything with
TYPE_USER where the actual input name is invalid (regardless of
underlying mangling for stuff like T31797).

Bug: T183211
Change-Id: I676642eee1222447df22a1c32b24f55e6273bcec