Add a granular capability for user directory browsing

Summary:
Fixes T4358. User request from IRC, but I think this is generally reasonable.

Although we can not prevent users from determining that other user accounts exist in the general case, it does seem reasonable to restrict browsing the user directory to a subset of users.

In our case, I'll probably do this on `secure.phabricator.com`, since it seems a little odd to let Google index the user directory, for example.

Test Plan: Set the policy to "no one" and tried to browse users.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4358

Differential Revision: https://secure.phabricator.com/D8112