Allow files to be deleted

Summary:
A couple of people mentioned that they've had users accidentally upload
sensitive files. Allow files to be deleted.

(At some point it might be nice to keep the file handle around and log who
deleted it, but this addresses the immediate problem without needing too much
work.)

Test Plan: Deleted some files.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T780

Differential Revision: https://secure.phabricator.com/D1423