Fix split_test xblock so that it can render in Studio

This change addresses STUD-1403 where split_test modules could not render in Studio.
It changes Studio to pass a new parameter 'runtime-type' of 'studio' when rendering
 an xblock. In split_test, this will then cause all of the children to be rendered,
 rather than just the one that matches the conditional.

This is the pared down version of all the previous attempts, after the failed attempts to
introduce a new view. We should revisit this and decide how to handle rendering of xblocks
in Studio in general, so that they don't need custom code like this. For now, this seems
like the best option to fix editing in Studio.

@cpennington @cahrens Please review (yet again!).