This is intended as an amendment to #5674 as using M_UNKNOWN as the errcode makes it hard for clients to differentiate between an invalid password and a deactivated user (the problem we were trying to solve in the first place).
M_UNKNOWN was originally chosen as it was presumed than an MSC would have to be carried out to add a new code, but as Synapse often is the testing bed for new MSC implementations, it makes sense to try it out first in the wild and then add it into the spec if it is successful. Thus this PR return a new M_USER_DEACTIVATED code when a deactivated user attempts to login.
