Remove token after used or when expired

This commit will close issue #96 

1) Remove token after first use
2) Remove token not used after first use > 24h
3) Semi rollback of previous commit: we must check matricola and token when signup/reset so they have to be in knownKeys. Instead the removal of hash was correct
4) Some mini debug text improvement

Tested on live Highgarden

Last time I tested the workflow of backend.hasValidKeys() and it worked without matricola in knownKeys. Unfortunately I haven't tested well backend.writeStudentInfo(newInfo). In general newInfo is an object with a matricola and other info to write. backend.writeStudentInfo() check the regexp before writing.