Course owners should be able to bump active period

[torgeirl]

Currently, updating a course's active period requires superuser access as it involves creating a new tag with category=semester (if it doesn't already exist) and then setting the course's active period to that tag.

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Course owner should be able to bump the active period of their course. When doing so, they should be asked to review administrators of the course.

Course admins should have the option to duplicate a course's assignments from one semester tag to another: #150.