LevelBuilder support for play lab

[Finishes: #79639696, #79639780, #79639834, #79639722]
There is a separate `editors/_studio.html.haml` with fields specific to Studio.  It makes sense for Studio to re-use Maze's handsontable editor to set initial character positions.  We talked at one point about making Studio and Maze both inherit from a common Grid class.  But there are only 2 fields that Maze uses and Studio doesn't so I think it's ok to special-case those 2 fields.