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. noted "Create Diff" button on filters

within user revisions and no button on filters within all revisions.

• entered another user into Select User UI and viewed their diffs via button and

pressing enter

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, btrahan, epriestley

Differential Revision: 1157