Deprecate redundant singleuser_ prefix on config

[minrk]

when configuring the Spawner, several traits have a singleuser_ prefix. This is mostly redundant with the fact that a Spawner is all about configuring a singleuser server. Most (probably all) of these should be deprecated in favor of the same name without singleuser_. The singleuser_ names can be kept as a deprecated alias from config to avoid breaking existing config.

[minrk]

The same probably goes for the few user_ traits, where user is a synonym for singleuser.

[yuvipanda]

I agree.