CYB-226 Engagement Index should be frequently, automatically, updated

* introduces sidekiq, clockwork, and clockworkd to do the work
* each of the two types (discussion and submissions) has its own async call
* adding a few scripts to help with the setup and maintenance

The initial setup and deploy decisions (on the coding side with e.g., foreman) or via IS&T with Linux init config is not yet determined.

[CYB-226 Engagement Index should be frequently, automatically, updated](https://jira.ets.berkeley.edu/jira/browse/CYB-226)