Unmisplace edit button in Diffusion

Summary:
It is misplaced from the beginning but it got worse after some CSS tweaks.

I was also thinking about using the same DropdownMenu as in Differential but I don't feel strongly about it to do it myself.

Test Plan:
Display file in Diffusion.
Repeat with disabled Editor.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1892