Run golden tests in CI (Resolves #36)

[matthew-carroll]

Run golden tests in CI (Resolves #36)

• Added Dockerfile to use golden_runner Docker runner.
  • TODO: Figure out how to not require a Dockerfile within each project that uses golden_runner
• Copied the golden matcher with pixel tolerance from super_editor to this package
• Added tolerance to SingleShot and Gallery but still need to add it to Timeline
• Fixed a bug where failure images were generated for passing goldens - problem was a missing return statement
• Fixed a difference between generated screenshots vs compared screenshots. Not entirely sure where the problem was happening, but the solution was to align the content left within the defaultGoldenSceneItemDecorator
• Regenerated all existing goldens using golden_runner so that CI should pass. One test required an 5px tolerance to pass GitHub's CI.

[matthew-carroll]

FYI @angelosilvestre - I'm gonna merge, but feel free to look through the changes/fixes.

We should try to come back ASAP and delete the Dockerfile, but that depends on changes to the golden_runner, which I have yet to publish to Pub.