Add problem history stub for Perseus exercises.

Auditors: eater, alpert
jQuery .trigger() expects extraParameters to be an Array

... when it isn't a plainObject.

Test Plan:
none, but this fixed the warning being uncloseable when I was mucking around with problem history.

Auditors: eater, alpert
Fix review nits

Fixes from rKEcbbd22da129cb8007ff95ff49fed08456856fa93.

Auditors: eater, cbhl
Remove custom stacks again again

Auditors: eater, cbhl
Merge remote-tracking branch 'origin/master' into perseus
Expose Khan.mathJaxLoaded for Perseus to hook onto

Auditors: eater, cbhl
Merge remote-tracking branch 'remotes/origin/master' into perseus