knowledge map tweaks

In the process of reducing from ~40 open branches to < 20, I found a few old kmap tweaks.  Some were superceded by @jamalex 's changes to reduce gaps in the map; others were still useful.  I've pulled it all together here.

Changes:
* Added a function to validate to remove missing "related videos", and code to validate that that works.
* Added relevant titles for each knowledge map topic page (instead of all having the same, generic title)
* Moved knowledge map tests into the khanload app
* Flipped axes explicitly in khanload, such that x now corresponds to h_position, y to v_position (thank goodness! :P)
* Moved @jamalex 's gap-reduction code into python / khanload, where it can be (pre-)computed and forgotten about.

Testing:
* Ran `./manage.py test khanload`, which pages through all knowledge map topic pages.  All look the same (including trigonometry, which is a big mess).

Confidence:
* High.  It works.  The code is relatively clean, with low-to-moderate levels of documentation.