Identify builtin files and give them open policies

Summary:
Fixes T7379. Currently, builtin files generate with a "users" view policy even if an install is public.

Because these files TTL after 7 days, there's no migration here. Installs won't see the fix actually happen for up to 7 days after updating, though.

Test Plan:

• Deleted a builtin.
• Loaded projects page to regenerate it.
• Saw new builtin had most open policy and was marked as a builtin.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7379

Differential Revision: https://secure.phabricator.com/D11917