CLC-2442 Handle primary section enrollments with same course catalog ID

https://jira.ets.berkeley.edu/jira/browse/CLC-2442

Many of us (including myself) had been told that a student could be enrolled in only one primary section of a "course offering" (department + catalog ID) & could only receive one final grade per semester for a given "course offering". We were misinformed. Departments give themselves more flexibility by defining a "course offering" which holds varied primary sections: multiple sports teams; multiple Phys Ed activities; multiple field studies; … And a student may take more than one of those primary sections (with different teachers) & receive different units & grades for each. 

This was pretty basic misinformation. As a result, many classes were not being shown correctly or were being skipped in GPA calculation. And also as a result, our source data doesn't provide everything we need for the ideal fix. Within our constraints, this commit should fix the problems students have reported. See the JIRA ticket for more background.

A couple of more minor fixes are bundled in as well:
- Don't unnecessarily stringify waitlist position & section enrollment limit.
- Don't count unrecognized/unavailable transcript grades as an "A" letter grade in the GPA Calculator.
- Don't do the intensive JavaScript work to set up an Academics semester page unless the browser is actually going to the semester page.