Consolidate grading/progress display logic

[ormsbee]

The frontend-app-learning repo implemented some rounding/bucketing logic for course grades on the frontend. #37399 pulls some of this logic back to the LMS backend for integrity reasons, however this PR also ends up replicating some backend grading logic for things like dropping the lowest scores.

Any duplication of grading code introduces the potential for drift between the score the user sees in their progress page and the scores that are actually being calculated for the user for the purposes of passing a course, granting a certificate, etc.

@pdpinch: Is this something that you folks could take the lead on?

FYI: @pdpinch, @sarina, @jmakowski1123, @arbrandes, @Anas12091101