Refactor and test Dynamath Code (tested, works; one more changeset coming, to add msg back into textinput_dynamath)

fix dogfood wrt lms / xmodule reorg; fix textinput_dynamath call

Removing references to django.conf.settings from capa/responsetypes.py

Problem testing utility, reducing courseware/capa dependencies on Django

fix breakage in problem checker caused by LoncapaProblem taking a file object in its constructor instead of a filename

customresponses can't tell us what their answer is (there can be a
wide range of answers -- it's verified by a small program). So here
we simply ignore those fields we can't generate answers for when
doing tests against problems.

Update documentation for functions inside common/xmodule