Integration of up-to-date khan-exercises codebase, plus some related fixes

This will be useful to look at on a commit-by-commit basis.

Changes directly related to integration:
* https://github.com/jamalex/ka-lite/commit/4eeba37ca5ad34cf646860b190d6e7f389eea20a
* https://github.com/learningequality/khan-exercises/commit/78e2836627e3c4c31300ead9667a7590c78ca111 (on our new local fork of khan-exercises, so we can integrate changes to khan-exercises in a versioned way)
* https://github.com/jamalex/ka-lite/commit/9db109e65845f1566bde17d6805a12e3feb56983
* https://github.com/jamalex/ka-lite/commit/cb96602b826d0f672439136fca98abe906a96cf7

Fixes for pre-existing bugs discovered along the way:
* https://github.com/jamalex/ka-lite/commit/65dbfd5e993466fc23d7d6118294eb75fdf24d48 (hiding the related videos when they're not available)
* https://github.com/jamalex/ka-lite/commit/1bbdbe6e102e9030515223c1fa580fc554e27c81 (this was a confusing bug; turns out some exercises exist in multiple topics, and their h_position/v_position coordinates were being flipped multiple times, with normalization happening in between, screwing up the knowledge map pages, e.g. for probabilities)

Might be good to have a quick skim over the code, but testing-wise, let's just get this into develop so we can pound away on it while doing our other work.