R1:aeae0e702830

Prepare Projects schema for subprojects

Summary:
Ref T10010. This does some cleanups on the schema:

- `viewPolicy`, `editPolicy` and `joinPolicy` were nullable, but should never be `null`. Set them to defaults if they're null, then make the column non-nullable.
- Rename `phrictionSlug` to `primarySlug` and stop adding and removing trailing slashes from it.
- Add new columns to support milestones and non-milestone subprojects.
- Drop very old subprojectPHIDs column. This hasn't done anything in…