Single repo implementation

[mansona]

This PR moves us into a "single repo" mode just like the new Ember Cli Guides project. This has a number of benefits over the two repo process we have been doing for the last year:

• We now have the ability to preview each PR and see how it would look before we merge (you can see this with the Netlify test app below)
• removes the need to deploy the guides-source to npm and then merge that change into guides-app
• any time we merge a PR into master it will automatically be deployed
• vastly simplifies the contributing guide for anyone who wants to run the guides locally while editing (no need to do any repo linking)
• we are now using actually shared implementations for the Guides and the Cli Guides, and will be using the same for the upcoming Tutorials app too 👍

It's also worth noting that merging this doesn't make it go live right away. I wouldn't merge this PR unless we have decided that we intended to go live realatively soon, but we have time to figure things out for a little while before going live.

[jenweber]

Note that we will need to take care of a couple odds and ends before we turn on Continuous Deployment. Good to see that merging !== CD right away.

[MelSumner]

can you clarify what this is combining? I have strong objections to merging the guides-source and guides-app- for maintenance reasons, I think it's strategically important for us to separate the content from the presentation layer.

[MelSumner]

@mansona looks like there's some conflicts to resolve first.