R1:70053beeed60

Smooth out milestone creation workflow

Summary:
Ref T10010.

- Default name to "Milestone X".
- Remove policy controls, which have no effect.
- Don't generate slugs for milestones since this is a big pain where they all generate as `#milestone_1` by default (you can add one if you want). I plan to add some kind of syntax like `#parent/32` to mean "Milestone 32 in Parent" later.
- Don't require projects to have unique names (again, 900 copies of "Milestone X"). I think we can trust users to sort…