Limit simulation time per user

[fabianishere]

At the moment, the hosted OpenDC service does not place any restrictions on the amount of simulation time consumed by their experiments. In case of a malicious user (or misconfiguration), this could lead to one user consuming all compute resources available for the hosted service.

This issue proposes adding limits to the amount of simulation time that can be consumed per user per time period (e.g., month). In this way, abuse or misconfiguration should be prevented.