Combined question type display edge cases. #11869

The problem is with question texts like
* Classify: Cat [[gs:selectmenu:1]], Dog [[gs:selectmenu:1]].
* Tricky [[1:selectmenu:1]][[2:pmatch]][[1:selectmenu:1]].

where the code for replacing the subquestion token like
[[1:selectmenu:1]] with the rendered question was getting it wrong.

The code also has to handle the case where the student inputs a response
that looks like a subquestion token.