Merge profilequestion type into module

There is no real good reason to use a separate content type for profile questions. This is proven by the client implementation which effectively turns profile questions into modules when generating a survey tree.

To simplify the implementation and allow better ordering of profile questions and modules we should merge the two content types into a single type. This needs a number of steps:

* replace the 'optional' flag for a module with a new flag to choose between 'optional profile question', 'repeatable profile question', 'optional module' and 'mandatory module'
* add a migration step to replace all profile question instances with module instances
* update the client to check use new module types instead of looking at content type

@cornelis The first step needs some UI thinking - perhaps it should be presented as two separate questions instead of a single four-option question. Your option is highly desirable.