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 the UI for years and years, and isn't particularly realistic to migrate forward.

The new columns aren't reachable from the UI.

Test Plan:

• Applied patches.
• Grepped for phrictionSlug.
• Grepped for subprojectPHIDs.
• Created tasks.
• Edited tasks.
• Verified existing tasks still had primary slugs.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14825