Petri renderer refactor

[mwdchang]

Summary

Clean up the petrinet renderer to prep it to be used for exporting to images.

• Removed CSS variable references as they are a nuisance to get right
• Removed edge interactions, this was back when we were editing edges manually and is no longer a viable route
• Clean up variable css file

Testing

Model diagrams for both stratified and unstratified models should look more or less the same as before.