[CI] Add Danger

[hramos]

Testing Danger support in CI. Continuation of #14959, which Circle stopped building.

Test Plan

Update your node modules first: npm install

Test WIP, package.json warnings

npm run danger pr https://github.com/facebook/react-native/pull/14951
Verify output. This PR should trigger a WIP warning, as well as a package.json warning:

> react-native@1000.0.0 danger /Users/hramos/git/react-native
> node ./node_modules/.bin/danger "pr" "https://github.com/facebook/react-native/pull/14951"

{
  fails: [],
  warnings: [
    {
      message: ":construction_worker: Work In Progress - <i>Do not merge yet.</i>"
    }, 
    {
      message: ":lock: Changes were made to package.json - <i>This will require a manual import. Once approved, a Facebook employee should import the PR, then run `yarn add` for any new packages.</i>"
    }
  ],
  messages: [],
  markdowns: ["This PR requires attention from the @facebook/react-native team."]
}

Test plan warnings

npm run danger pr https://github.com/facebook/react-native/pull/14946

Verify output. This PR should trigger a warning against the lack of a test plan (note that the PR does have a test plan, but it does not title it as such):

{
  fails: [],
  warnings: [
    {
      message: ":clipboard: Test Plan - <i>This PR appears to be missing a Test Plan</i>"
    }
  ],
  messages: [],
  markdowns: []
}

Warn when a change to the Getting Started guide is missing a test plan

npm run danger pr https://github.com/facebook/react-native/pull/13186

Should warn against a missing test plan:

{
  fails: [],
  warnings: [
    {
      message: ":clipboard: Test Plan - <i>This PR appears to be missing a Test Plan.</i>"
    }
  ],
  messages: [],
  markdowns: [":page_facing_up: Thanks for your contribution to the docs!"]
}

Flag PRs by core contributors

If the author is able to issue bot commands, we reasonably assume that this is coming from an established core contributor. Their PRs will be flagged for expedited review:

npm run danger pr https://github.com/facebook/react-native/pull/14895

{
  fails: [],
  warnings: [
    {
      message: ":clipboard: Test Plan - <i>This PR appears to be missing a Test Plan.</i>"
    }
  ],
  messages: [],
  markdowns: ["This PR has been submitted by a core contributor. Notifying @facebook/react-native."]
}

[pull-bot]

	Warnings
⚠️	👷 Work In Progress - Do not merge yet.
⚠️	🔒 Changes were made to package.json - This will require a manual import. Once approved, a Facebook employee should import the PR, then run yarn add for any new packages.

This PR requires attention from the @facebook/react-native team.

Generated by 🚫 dangerJS

[hramos]

Testing mentions: @facebook/react-native

[charpeni]

That's great!

[hramos]

Mentions aren't working well when used within Danger's markdown() call.

[hramos]

This is necessary due to danger/danger-js#261, and can be removed once Jest releases 20.1.0 or danger-js updates its dependency on jest to 20.1.0-alpha.3.

I don't want to merge this PR until we can avoid using this workaround.

[orta]

The mentions issue may be because the bot cannot see the group. I cannot use that either:

[hramos]

Ah, the team is only visible to organization members. As we don't want this GitHub bot to be a member of the Facebook org, looks like I won't be able to leverage mentions here.