Taking Ike's delete-student-state-for-problem code and putting it in a separate PR

Taking Ike's delete-student-state-for-problem code and putting it in a separate PR so it's not bundled with more debated features.

Non-developer docs for course staff and researchers

Feature/brian/pdfchapter

Add in rubric hotkeys

Jth/fix draganddrop

Fixed an error in symmath_check

the expected result = 0.  symmath_check() was dividing
a quantity by the expected result, leading to a NaN value
that caused the answer to be marked incorrect.

Added test_symmath_check.py with basic tests for
the symmath_check() function.

Somehow the saves changed to just sets w/ don't send data to the server.