R1:2044e5120625

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…