Specifying the class period for student imports #75

Currently, there's no easy way to automate assigning students to periods. That's why some classes on Tin have stopped using the periods feature, despite how helpful it can be.

One solution would be to let teachers assign StudentImport objects a period. When a student logs in, they'll be added to the course as normal, but if the StudentImport object also has a period, they'll be auto-assigned to that period (which can be created if necessary). Then, teachers would ideally create a StudentImport object for each class, and Tin can take care of the rest.