feature: login (for access restrictions, single user assurance)

[stefanix]

• provide a simple login (optionally with user quotas)
• one active login at a time, prevent two people from using the lasersaur at the same time
• keep login alive pulse so that lasersaur becomes available if somebody forgets to logout
• restrict access to admin tasks like flashing firmware

[stefanix]

we have a ready status now ... so at least a second user cannot mess too much with a running job