make the numbas script loader work with new-style loadScript

A while ago I changed the way loadScript works in the main Numbas runtime so that files are named without the full path or .js extension, since they're all being concatenated together. There's also a "base" dependency which the runtime uses to make sure that non-Numbas libraries like jquery and knockout have loaded.
The editor still loads files individually and on demand, though, so I left everything the old way, which meant I had to change the loadScript line each time I copied a new version of a Numbas script over.
This modifies the script loader so it rewrites new-style names to old-style paths, meaning the copied-over scripts can be identical to the ones used by the runtime.

Really, I should add a manage.py script to concatenate all the files used by the editor...
update math.js
don't load currentPartTab if currentPart is null