[Continues 68414526]: add minWorkspaceHeight

The blockly workspace height is determined by the combined height of the two divs to its left: visualization and belowVisualization. Normally, the auto-sizing is cool. The height isn't too tall for simple levels (guaranteeing it will fit better on small screens, tablets, etc.). And when we fill the belowVisualization area with video thumbnails, we give the user the extra workspace height "for free".

However, for some complex levels, we know that we want the blockly workspace to be a minimum height - either because the toolbox has lots of blocks or because we expect the solution to take a lot of space. This enables that capability on a per-level basis for studio. It could be easily adapted to the other apps - or perhaps merged into the BlocklyApps "base class" at some point in the future.