Expose all users in the UserResource for public LocalSites.

The addition of public local sites exposed a bunch of assumptions in the user
linking. By default, the user resource within a local site only contains those
users who are actually members of the site. This causes a bunch of bizarre bugs
in rbtools or other consumers of the API, because the links between objects
(such as the review request submitter) could be 404.

This change makes it so that if a local site is public, all users on the server
are exposed in the site's user resource.

Testing done:
Verified that links no longer 404 using cURL. I intend to add a unit test for
this when porting to 2.0.x (since the API testing framework there is much
improved over 1.7.x's)

Fixes bug 3484

Reviewed at https://reviews.reviewboard.org/r/6110/