Add "Resign from Audit" and "Close Audit" actions to Diffusion

Summary:
See some discussion in D2002. Add two new actions:

• Resign: (auditor only) closes your open request (user request ONLY) by putting it in a "resigned" state.
• Close: (author only) closes all open requests by putting them in a "closed" state.

@davidreuss, this is probably conflict-city with D2002 -- I'll wait for you to land first and then handle the merge on my end.

Test Plan: Resigned from and closed audits.

Reviewers: 20after4, davidreuss, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D2013