Migrate existing projects to retain "Workboard" as default item

Summary:
Ref T10054. Ref T6961.

• Existing projects with workboards had "Workboard" as the default menu item. Retain this behavior.
• Populate the recently-added hasWorkboard flag so we can do a couple of things a little faster (see T6961).

Test Plan:

• Ran migration.
• Verified a bunch of projects looked sensible/correct after the migration.
• Created a workboard, verified hasWorkboard got set properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6961, T10054

Differential Revision: https://secure.phabricator.com/D15093