R1:bb633fb42afa

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

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