Added mode option to course enrollment management command

This data can be used by external frontends or other apps. The list
of course enrollments can optionally be filtered by course id or
usernames.

EDUCATOR-3796

EDUCATOR-3796

EDUCATOR-3796

ARCH-248

Note: this version update is backward incompatible.

ARCH-255

LEARNER-6469

This reverts commit 370501d1.

LEARNER-4744

LEARNER-5166

Django 1.10 deprecation fix for Hackathon XIX
Addresses PLAT-1397

LEARNER-5287

LEARNER-5038

This reverts commit 813d59d0.

LEARNER-5038

- Prevent RetirementState state_execution_order collisions when updating
- Add slash to end of unenroll API endpoint to match other retirement endpoints

URL patterns changed to follow edx API conventions

https://docs.djangoproject.com/en/1.11/releases/1.10/#using-user-is-authenticated-and-user-is-anonymous-as-methods

LEARNER-5101: The current rate limit was adjusted to be
very high for staff users to account for excessive calls
from the ecommerce service. The ecommerce service has
been updated to not make these calls. This commit intends
to adjust the rate limit to it's original state before the
excessive calls.

[LEARNER-4676]