New Event Proposal: `SUBSECTION_GRADE_CHANGED`

Modular Learning work (Competency Based Education for now, but also Learning Pathways later on) would benefit from having a learner SUBSECTION_GRADE_CHANGED event that would be the subsection counterpart to PERSISTENT_GRADE_SUMMARY_CHANGED. This would allow us to build more of this new functionality inside Learning Core, rather than edx-platform.

@mgwozdz-unicon, @mariajgrimaldi, @Agrendalath: This is just a stub for now, but I plan to fill this out into a real proposal later this week. Just wanted to give you folks a heads-up. This would be strictly additive, so hopefully it'll be straightforward and uncontroversial.