switch to using jQuery; refactor jQuery var into const

I needed this because we have Prototype installed still but we can't get rid of it that fast. For the moment, we need jQuery to be accessed using `jQuery` and not `$`.

I could use some help determining where best to initialize `JQUERY_VAR`. Right now it will be defined when the gem/bundle gets loaded.

Also--are you concerned about tests for this repo? Lots of (seemingly old) failures, so I didn't fix anything.