Add more static references for getting to js webjars

This probably isn't a blocker, but it feel like it would be really nice to get into 11.0 and would make a lot of things easier. Basically we've got this pattern that I didn't know about with editor-tool (/sakai-editor/editor-launch.js) that you can call and it gets you the most recent version of the editor.

This works because the
portal-service-impl/impl/src/java/org/sakaiproject/portal/service/EditorRegistryImpl.java

Says which version of the editor is installed 2 things that would be nice.

1) A way to register any javascript that's installed from a webjar, not just editors.
2) A way to define an override version of the file so it could be upgraded and maintained externally.

I think the generic case of webjar registry could be patterned off this editor registery but should be separate.