New sections sheets to replace section sheets per term

We decided to split up the table into Sections, SectionEnrollment and SectionTimetables sheets because these tend to be separate steps anyway. It results in less crowded sheets as well. Another change is to no longer accept blank cells even for repeating cells like school year id because it only confused users more than it helped. Another benefit of requiring the cells is that it helps when using the sort function from within an xls file.