Update RBServerity to use the new JSExtension support.

Review Request #5530 — Created Feb. 23, 2014 and submitted — Latest diff uploaded

Information

rb-extension-pack
master
eac7b65...

Reviewers

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.