Implement DangerJS

[mxstbr]

Danger is a bot to codify coding norms. Not like prettier for code, but for the humans.

This PR sets up Danger, and sets it up so it does a couple things:

1. Post test failures as a comment in a GitHub PR so you don't have to have to open Codeship to debug builds
2. Make sure you added a description to the PR
3. Make sure you updated the yarn.lock file if you added or removed a dependency in any folder
4. Makes sure you don't leave .only on a test and only that test runs

It's very handy, and sets us up well for the future when we go open source. We've used it to great success at styled-components, and it's worth adding here too.

NOTE: I don't yet know if this works, consider this a test case 😅

[mxstbr]

It worked! This is what it showed when the tests failed:

😍

[brianlovin]

Cool! Besides the codeship build failing because of e2e timeouts (i just restarted it) this is awesome