R1:519a443eba70

kill differential tabs in favor of a create diff button

Summary:
kill the tabs and make it a create button instead. pertinent notes:
* added a "Filter diffs" button to the form. optional, but i thought it
necessary with the new green button
* linked to Arcanist user guide on the create diff page. somewhat unrelated but
i think create diff will get more traffic now so linking to help seemed like a
reasonable add on here.

Test Plan:
viewed differential homepage
* clicked left hand filter elements.