Fix an overlap between two elements of the Image form input.

Summary:
The filename field and the checkbox to select the default image were
overlapping in Firefox on Linux on both the Project Edit page and the
Profile Edit page.

Test Plan: Looked at both of the pages and saw that they rendered better.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3079