Fix formatting for linter

Fix order to be alphabetical

Surface engagement metric ranges according to spec

AN-6899

Better handles edge cases when there's sparse data.

Upgrade django debug toolbar

Added documentation to learner api for account url.

Upgrade to Django 1.8.13

Upgrade elasticsearch-dsl.

* this will in turn restrict the elasticsearch library to a compatible version
* added unit test for unicode character cohort filtering

Moved default load data options to make_option.

Learner analytics

Fix the way we measure coverage

AN-6960

Now distinguishes between metrics which are aggregated by unique entity
id (such as problems or videos) and those which simply add up the total
number of interactions, regardless of which module they were acted upon.

- discussions_contributed renamed to discussion_contributions

AN-6318

Authors:
- Dennis Jen
- Daniel Friedman

AN-6158

Also removes the 'problem_attempts' field from the roster entry, since
there's no defined use case.

AN-6151