Sim Feature: Can start/stop the graph sim from running

[devonh]

The visjs graph can consume a lot of processor time when simulating large networks. This results in two potentially undesirable behaviours.

1. simulation state update messages taking longer to be processed as they are received
2. processor usage gets really high

It would be beneficial to be able to "pause" the visjs simulation. This would greatly reduce the processing currently happening and allows the sim ui to quickly process new updates. Visjs already has this ability in the API, the sim just needs some added UI and to have it hooked up.