R1:389e4d1b1fa0

Lock milestone projects to an automatic color/icon

Summary:
Ref T10010.

Currently, milestone subproject have editable icons/colors, but I don't think this is likely to be used much (the expectation is that milestones will be common and homogenous, and it doesn't make much sense to pick different icons for "Sprint 32" vs "Sprint 33", I think).

Locking the icon and color lets us simplify the form, make milestones more distinct, and potentially reuse the color later for other things (e.g.