Make Harbormaster input and output artifacts more explicit

Summary:
Ref T1049. In Harbormaster, build steps may have various inputs (like a host they should run on) and outputs (like a reference to an uploaded file).

• Currently, inputs aren't defined anywhere (except implicitly at runtime).
  • Instead, define inputs explicitly.
• Currently, outputs are defined in a way that loses information when misconfigured (the keys will collide).
  • Instead, define inputs and outputs so they work whether a step is configured correctly or not.
• Currently, there's no simple way to see a step's inputs and outputs.
  • Add some UI for this.
• Currently, reordering steps has some surprising side effects.
  • Instead of invalidating steps after reordering them, validate them at display time and warn the user.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, chad

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8599