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

Also remove useless docstrings where they were added to keep pylint
quiet.

* Add verification deadline model.
* Populate verification deadlines from course modes table.
* Update student dashboard to use verification deadlines.
* Update pay-and-verify view to use verification deadlines.
* Simplify Django admin for course modes and add validation.
* Add verification deadline to Django admin for course modes.
* Add UI for when the upgrade deadline is missed in the pay-and-verify flow.

* Adds a credit course mode to indicate that a course
has a credit option.

* Hides the credit option from the track selection and
pay-and-verify pages.

* Shows different messaging for the verified track if
it's possible to upgrade from verified to credit at the end
of the course.

ECOM-1228

[LMS-2757]