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 this out for themselves since modern Phabricator has "Can Create Projects" permission, etc.

Test Plan: Created some milestones, had a less awful experience.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14909