Add datadog instrumentation

[TIM-305](https://edx-wiki.atlassian.net/browse/TIM-305)

This generates events for all of the metrics described in the JIRA ticket, except for tagging turbo grading.  I didn't see anywhere this was exposed to the peer assessment API, and I'm hesitant to change the interfaces this late in the game.

This also cleans up some technical debt: the assessment API no longer uses the `Submission` and `StudentItem` models directly [TIM-404](https://edx-wiki.atlassian.net/browse/TIM-404)

@stephensanchez @feanil