Block new registration attempts

[kashalls]

Some users are going to want to self-host this, in the event that it is exposed to the Internet, there should be an environment variable that can be passed to disable new registrations of devices/accounts. Most likely this would be helpful to allow current users to add more JetKVMs, but also block other users from registering their JetKVM.

[nslsrv]

Hi. Not fully covering the use case that you've described, but there's a merged feature to explicitly restrict userlist, that might be helpful to some of the subscribed users.