Levelbox clicking mostly works

Levelboxes are hooked up! For the most part. The following still threw
errors or didn't work properly:
* Clicking on ability[1].levelbox[1] to fill it ("can't read property
'cost' of undefined" error, from client/controllers/skilltree.js, line
84).
* Clicking on ability[3].levelbox[1] to unlock it ("unlock previous slot
first" error response, so something in the computational checks is
borked).