Add very basic UI for creating milestones and subprojects
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).
- 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 members).
- Observed plenty of silly/confusing interactions that need additional work.
Reviewed By: chad
Maniphest Tasks: T10010
Differential Revision: https://secure.phabricator.com/D14904