begin CI integration for bots

[jbaxleyiii]

This PR brings support for a number of automated bots to help with issue triage, repo management, and code reviews in Apollo Client.

A few to note:

• Danger (shown below) can take the place of the checklist template. It will perform the required checks for you and fail / warn / message anything needed
• bundlesize is a new status check replacing the hacked together method I wrote a while back. Its great!
• probot is like hubot for github. It will unify labels across our projects, help close stale issues, and provider friendly welcomes for issues!

If any of these bots become cumbersome, we can remove them or change them. The goal is to help maintainers and contributors work together more easily and assist in some of the smaller jobs where possible.

[mention-bot]

@jbaxleyiii, thanks for your PR! By analyzing the history of the files in this pull request, we identified @stubailo, @helfer and @shadaj to be potential reviewers.

[apollo-cla]

	Messages
📖	Please add your name and email to the AUTHORS file (optional)
📖	If this was a change that affects the external API, please update the docs and post a link to the PR in the discussion

Generated by 🚫 dangerJS

[stubailo]

Whoa this seems dope. let's do it!