R1:96e3c73159e6

Put "Subprojects" on top of "Milestones" in the Project UI

Summary:
Depends on D19550. Ref T13164. See T12144#226172, mostly. We get some requests to make milestones reorderable, but in most cases users probably wanted subprojects, not milestones.

One reason to end up here is that we put "Milestones" on top. Instead, put "Subprojects" on top, since they're the less specialized option and we aren't terribly consistent about it anyway.

Test Plan: Viewed project subprojects page, saw "Subprojects" above…