Show "Search" in menubar while logged out if users can access it

Summary:
Ref T603. If an install allows acccess by logged-out users, show search.

(A lot of the search typeahead results, although visible to the user, don't lead anywhere interesting right now. We can clean this up in the future.)

Test Plan: As a logged out user, searched for some stuff. It worked. Also, I only found results I could see, which is quite heartening.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7153