Add a "Create build step" transaction to Harbormaster

Summary:
Without this, build steps that have no options (like "wait for previous commits") don't actually save, since the transaction array is empty.

This also generally nice and consistent.

Test Plan: Created a new "wait" step, viewed transaction log.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8791