Analytics updates

This has a few different updates for analytics:

* Adds original timestamp to queued analytics events, so that even if sidekiq goes down and the queue builds up, once sidekiq is restarted and processes the queue, the analytics events will log with their original event queued time instead of the processed time.

* Adds student_id to analytics data for logins, pageviews, tab-selects, and predictors.

* Installed Sidekiq monitoring dashboard, which is accessible at the /sidekiq URL as long as you're logged in as an admin.