CLC-3697 GPA Calculator should omit ungraded past enrollments

https://jira.ets.berkeley.edu/jira/browse/CLC-3697

Includes both fixes in the JIRA description:

* Do not include old enrollments for which there is no official final grade in the GPA Calculator. (And *especially* don't assume that they should be treated as an "A" in the cumulative GPA!)

* Immediately after the semester ends, during the three-week period in which grades are finalized, show the final grades that are available but continue to treat any ungraded enrollments as "estimated" grades.