Add actAsUser to API

Summary: createrevision creates the revision as the user which certificate is
used. Add a meta parameter to API calls named actAsUser so one user can create
revisions for someone else. Right now there is no authentication.

Test Plan: Called createrevision with one users cert and set actAsUser to
someone else. The revision was created as the actAsUser user.

Reviewers: epriestley, nh, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1087