Remove partner content, update topic tree, and fix a couple of exercise bugs

Sorry for the mish-mash of related changes. Probably easiest to review this commit-by-commit.

* Removes "partner content" from the topic tree (didn't have CC licenses)
* Fixes up some formatting on the frontpage caused by updated topic tree.
* Fixes a bug also fixed in the unmerged #472, to get quite a few exercises working again
* Adds some images needed for one_step_equation_intuition exercise
* Fixes the positioning of the CC logo in the bottom