Update RBServerity to use the new JSExtension support.
Review Request #5530 — Created Feb. 23, 2014 and submitted
Review Board 2.0 beta 3 replaced js_model_class with JSExtension. This
is a simple change that updates RBSeverity to use it.It also switches to using initialize and apply_to.
Tested that the JavaScript was loading on all the relevant pages.
