Bszabo/tnl 10136 student course enrollment

[bszabo]

This is an incremental step towards teasing apart a course enrollment model out of the existing student model in edx-platform. These changes are intended to be non-breaking. Please fail the PR if that objective goes unmet.

Model code had previously all resided in a single student/models.py module. Now it resides in two modules, one for student modeling and one for course enrollment modeling.

Database tables remain as is.

Importing of student and course enrollment models remain as is.

[connorhaugh]

Great use of atomic commits here it makes reading the story of this revision much easier.

[bszabo]

In side channel slack conversation, Alexander confirmed that none of these changes broke the edx-enterprise build, and that the extraction of the course enrollment from the student model looks largely correct (suitable first approximation).

[connorhaugh]

@bszabo If you could hi light any changes required by the merge, I would be happy to give this a second thumb upon review of those changes.

[e0d]

Hi @bszabo should this PR be closed?

[feanil]

Closing since there hasn't been a response in a while.