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.

Feature/victor/revert revert of forums ui

Fixed a bug in symmath_check():

add a 'missing slash' regular expression for Location patterns in order ...

evaluated bool(ans), which is False when ans=0.
This led to an error message "[symmath_check] failed to get MathML for
input; dynamath=[None]" when the function reached a later condition.
The check that evaluated  bool(ans) was redundant, so I removed it.

add a 'missing slash' regular expression for Location patterns in order to account for dropped slash with the i4x:// pattern. Certain redirect cases will cause this 2nd slash to be dropped (I think via Nginx)

Feature/victor/dynamic cohorts

Have to return false from event handlers to avoid href="#"
links from sending the user to the top of the page.

See changes in xml-format.md or wiki (dynamic cohorts v1) for spec.

Add showanswer="finished" option and tests