added missing backend tests for users api

- functions include create/edit users, update password, get course
list
refactored test data

- instead of reproducing some data like CriteriaAndPostsForQuestions
in several test data classes, I have now restructured some of the
test data so that they can inherit other classes' object
- fixed criteria backend test which broke during the data restructure
- refactored function judgement_count in the judgement model, so that
it can be more efficient. Also it now uses the same "algorithm"
as the gradebook
- side note: in some of the committed files, the tabs have been converted
into spaces.
created backend tests for the comments api

- I've created backend tests for:
	- comments for answers
	- comments for questions
- removed simple comment tests from test_acj
- created test data for the comment tests