Callouts: initial support [finishes #58916350] [related to #60345746] [unblocks #59139064]

* Add initial tests for `ScriptLevelController`
* Ressurect some of the old callout / qTip code (removed in series of commits October 2013)
* Makes callouts ScriptLevel specific (by `Script.id` and `Level.level_num` during import)
* Adds `cdo` namespaced `events` jQuery object and `eventType` enum
* Sample callout just shows on first modal hide, removes on any click