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:
{F133679}
{F133680}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, chad

Maniphest Tasks: T1049

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