Provide new tool "bin/user approve" to approve an account.

Summary: T13578

Test Plan:
This method uses the existing transaction. As such, most of the testing focused on the integration between the workflow and transaction. The only change made to the transaction was to allow an omnipotent user to make the change in addition to an admin.

Other than that, I removed the "approved" flag from the user, then ran the command-line utilty until the user was successfully approved.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T13578

Differential Revision: https://secure.phabricator.com/D21587