R1:7c5ad63fd1d6

Add very basic UI for creating milestones and subprojects

Summary:
Ref T10010. This has a lot of UI/UX problems but I think it:

- technically allows subproject creation;
- technically allows milestone creation;
- doesn't let users unwittingly destroy their installs (probably).

Test Plan:
- Created milestones.
- Created subprojects.
- Created and edited normal projects.
- Observed some reasonable interactions (e.g., you can't create milestones for a milestone or edit a superproject's…