Make registration authority-specific or even election-specific

[andres-erbsen]

A bad election authority could deliberately confuse multiple registration requests that differ in the LSAGS public key, thus creating a plausible explaination for not counting that user's vote. To avoid this,

• authorities should be allowed to allow each user register only once
• one authority must be unable to interpret a request to register with another authority as a request to register with them.

Fix: add the authority's public key fingerprint (or equivelent) to the registration request.