Edx-organizations

We needed an interface to bulk activate/deactivate organization-course linkages. This version implements one in the Django Admin panel, based on the existing admin action for activating/deactivating organizations.

Also, we add history tracking to the OrganizationCourse model. Unfortunately, changes made via bulk edit will not be reflected in the history table due to limitations in Django ORM's bulk-write API.

For screenshots, see https://github.com/edx/edx-organizations/pull/169

Fixed a bug where Course Discovery could not push organization updates to LMS/Studio if the organization already existed there but was inactive.

Specific changes to the `v0/organizations` API:
1. Any organization may be updated via the PUT method, regardless of whether or not it is active.
2. Upon being updated, the organization is marked as active, whether it was previously active, inactive, or non-existent.
3. The 'active' field may not be set or updated via the HTTP API, because that would be in conflict with change 2.

See https://github.com/edx/edx-organizations/pull/165 for details.

Package Updates:
- Dropped support for python3.5
- Added tests for Django3.0 & Django3.1.

Package Updates:
- Dropped support for python3.5
- Added tests for Django3.0 & Django3.1.

Add a new optional flag `activate` to both:
* `api.bulk_add_organizations` and
* `api.bulk_add_organization_courses`,
defaulting to `True`, which is the current behavior.

With `activate=False`, those functions will set the `active` flag on newly created objects to `False`. Furthermore, they will not re-activate any existing objects. The purpose of this is to allow us to backfill the Organization and OrganizationCourse tables on production instances without introducing any new active data. We intend to expose this flag as a command-line flag in the edx-platform `bulk_add_orgs_and_org_courses` command.

This release also upgrades testing requirements and makes some minor code changes.

No breaking changes.