LevelBuilder support for play lab

[Finishes: #79639696, #79639780, #79639834, #79639722]
This is just a comment that describes how the routing works (eg. this method is called when you send a PATCH/PUT request to the url /levels/1). Comment doesn't actually 'do' anything -- the routing is actually defined in config/route.rb. It's common in rails apps because the scaffolding generator generates them, but I sometimes put them in myself when the routing is weird or non-default (this happens to be the default routing).