Clean up the Diffusion search UI a little bit

Summary:
Ref T156. @vlada recently implemented filename search in Diffusion, this cleans up the UI a little bit:

• Instead of showing one search box with two different buttons, let the submit buttons appear to the right of the text boxes and separate the search modes.
• Clean up the results a little bit (don't show columns which don't exist).

Test Plan: {F107260}

Reviewers: vlada, btrahan, chad

Reviewed By: chad

CC: vlada, chad, aran

Maniphest Tasks: T156

Differential Revision: https://secure.phabricator.com/D8125