Python leaks variables from list comprehensions (Why, Python, why?) which breaks any later attempts to use the path module

Dogfood

Conflicts:
	templates/mathjax_include.html

Sequence content does not double-escaped anymore, and eval() usage was
also removed from the sequence module.

Kf minor ui

Fix Youtube player glitches on iOS

Conflicts:
	djangoapps/courseware/capa/responsetypes.py
	djangoapps/courseware/module_render.py
	djangoapps/courseware/modules/html_module.py
	djangoapps/courseware/modules/seq_module.py
	djangoapps/courseware/modules/x_module.py
	djangoapps/courseware/views.py
	templates/main.html
	templates/problem.js
	templates/textbox.html

is completely contained within lib/dogfood, and it can be cleanly unplugged
from the rest of the code.

This prevent user from prematurely seeking the video.

This will fix the problem when user trying to click the seek button on
the iOS, as the video need to be started first before it can be
seekable.

iOS devices has a restriction that user has to click on the video
element to initiate playback. By visually disable the button, user will
be forced to click on the video itself, and everything will be worked as
expected.

Fixes https://www.pivotaltracker.com/story/show/30334381

for now

with external server into ExternalResponse.do_external_request

for getting expected answer from external server, add error handling,
extend to use json for multiple input fields, added rows & cols to
textbox inputtype for code input.

also some changes to try to fix the codemirror problem

cleaned up some cruft prints -> logging

symmath library; add options for matrices, qubit to symmath_check

after click due to call to content_updated(); fixed this.

generated when Check is clicked)