Support constructing passwords via HTTP REST

[domob1812]

Light clients will not only need to read a current name's state, but they will also need to construct authmessages and passwords (after signing the message themselves). This can be done through libxidauth, but doing that might be hard e.g. for non-C/C++-based clients.

Thus it would be good to also expose getauthmessage and setauthsignature through the HTTP REST interface.

[domob1812]

Exposing those methods via the HTTP interface is not a good idea, as it will also expose the user's password to the server. Instead, we will do #8.