- switch to using registry
 - pull math and solution "input types" into a separate customrender.py file
 - make capa_problem use new custom renderers and input types registries
 - remove unused imports, methods, etc
 - add tests for math and solution tags.

- also add a test for chemicalequationinput

- tests
- rename it codeinput, with textbox still supported too

- add test

Unify math and non-math code and templates.

Add tests.

- Make it into a class.
- Combine ChoiceGroup, RadioGroup, CheckboxGroup implementation.  (All three tags still work--this just unifies the code)
- add tests

Fix imageresponse in Firefox

putting vsepr static image files where vsepr js can find them. TODO: reogranize after proper input type static file storage is sorted out.

Permit collapsibles in capa detailed solution

Add an end date to courses.

- needed for cert generation (and later lots of other things)

Vik/add instrumentation

Vik/add instrumentation