Add a PER_USER leasing strategy to policies

[ezekg]

To enforce process limits on a per-user basis. Should only be applied when the process's machine has an owner.

[ezekg]

When leasing strategy is set to PER_USER, max machines and max processes should both be per-user.

So it'd look like this:

• PER_LICENSE: max machines per-license, max processes per-license.
• PER_MACHINE: max machines per-license, max processes per-machine.
• PER_USER: max machines per-user, max processes per-user.

[ezekg]

How do we handle null machine owners? Should we add a requireMachineOwner attribute that must be enabled before a PER_USER leasing strategy can be used? Or should a null owner be considered a unique user? I lean more towards the latter because I'd like to avoid yet another attribute on the policy object.

[ezekg]

Some things that will need to be adjusted for the PER_USER leasing strategy:

1. Machine limits need to scope per-user, not just per-license, both during machine creation and validations.
2. Ditto for core limits.

These are not simple changes, but shouldn't be too complex.