* 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
* 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
