CLC-4410 Making calendar more scalable

https://jira.ets.berkeley.edu/jira/browse/CLC-4410

Rework Calendar::Exporter to use the Batch API, 500 requests at a time. The extensive use of Procs for callback is a bit new to our project but made a lot of sense given the flow of logic (and the callback-oriented setup of the Batch API). 

Don't merge this until after we've branched sprint 48 for QA!