Hide upload JS stuff if user isn't logged in

Summary: ...cuz it won't work. I think adding a "login to upload" has no real value as this is a pretty unexpected / power user feature anyway. Fixes T4354.

Test Plan: tried to upload as a logged out user to Phabricator home. my browser just loaded the file as expected.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4354

Differential Revision: https://secure.phabricator.com/D8298