…methods

We'll be using them in AuthHandler in the separate PR, so move them to AuthHandler
instead of importing them from LoginRestServlet

Additionally moves some checks that were in
`LoginRestServlet._do_other_login` to
`convert_client_dict_legacy_fields_to_identifier` as they would be
applicable to any other callers of this function.

Additionally, there is a functional change here, in that we now return
M_MISSING_PARAM's instead of M_UNKNOWN errcodes for when `identifier` or
`type` parameters are missing.

This function changes the errcode of a SynapseError from M_UNKNOWN to M_INVALID_PARAM.
I think it fits better, but this is changing behaviour...

Also corrected docstring. There is no 'm.id.threepid' identifier type.