LevelBuilder support for play lab

[Finishes: #79639696, #79639780, #79639834, #79639722]
As we discussed I think it would be best in the long run to include an intermediate class e.g. `Grid` since the Studio is not exactly a Maze-type level, which causes some confusion and necessary complexity (e.g. the need to disable 'start direction' and 'step mode' properties for Studio type at the top of editors/_maze.html.haml). I think it's fine to leave this for now if it works, but we should keep this in mind as the first refactoring step as soon as any further complications arise due to this down the road.