Don't create invalid build steps while adding them

Summary:
Ref T1049. Currently, the "add" dialog lets you select a build step type, but then immediately creates one. If you "cancel" from the edit screen, you end up with an empty (and almost certainly invalid) build step.

Instead, don't create the step until it's valid.

Test Plan: Add Step -> Pick Type -> Add Step -> Cancel no longer creates empty step.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8605