Resizable Panes

The resizable panes (that depended on Prototype) for the codeviewer/rubric for graders and students have been replaced with a jQuery-dependent version. This is based on [my own custom resizable panes](http://echeung.me/files/panes) and is designed so that the right column can't be pushed off the page like before.

Minor quirk also fixed along the way: refreshing the page when you're not viewing the first file no longer shows the first file again on load and instead shows the file you were on before.