Add Course Section metadata to support learning analytics

Although a few Course Formats like Weeks define start and end dates per section, we need the ability to define start and end dates per section outside of course format, so we can assume (lacking due dates on individual activities within sections) when those activities are expected to occur. By default, each section should have start and end dates calculated from the course start and end dates, divided equally among sections. The course creator/instructor should be able to go to an overview page and adjust individual section dates as needed.