EDUCATOR-4903. Move defaults to correct place - cr

* Check learner enrollment status to avoid mixing learners from incompatible tracks (e.g. masters/audit) to the same team
* When a new team is encountered on a team management CSV, create it with the protection status of the user

* wip

* get

* quality

* change oauth2 to deprecated?

* quality

* remove duplicate enrollment check function

* code review

This will remove imports from __future__ that are no longer needed.

https://docs.python.org/3.5/library/2to3.html#2to3fixer-future

* For all files, from __future__ import unicode_literals.
* Add @python_2_unicode_compatible to both models.
* Test six.string_type(obj) for both models.

Also, fix some new pylint warnings and improve model repr methods.

MST-24

MST-6

* run python modernize

* run isort

* Fix imports

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

TNL-3303

TNL-3112 TNL-3154

These signals are caught by the Teams app and used to update
`last_activity_at` on both teams and individual users.

TNL-2497

TNL-3068

TNL-3063

TNL-1911