Lockdown tool directory editing to administrators

Summary:
Someone has "defaced" secure.phabricator.com with a helpful suggestion that I
actually do this; fair enough. :P

Test Plan:
Logged in as myself, unable to edit directory information. Logged out, logged in
as admin, was able to edit directory information.

I need to fix some more CSS stuff since some of these tabs render out hideous in
the admin background, but I can followup with that.

Reviewed By: tuomaspelkonen
Reviewers: aran, jungejason, tuomaspelkonen
Commenters: aran
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 296