Makes gatsby files more readable and structured

closes #70

🚚 Make createPages API methods more readable

Splits the functionality in gatsby/createPages.js into three different exported functions:
1. createRedirects
2. createGhostPages
3. createMarkdownPages
Return those three functions in form of Promise.all() in our gatsby-node.js file instead of simply requiring the whole file.
This way we are able to see what's meant to happen within our createPages without the need to open the required files, as the methods are explaining the functionality themselves.

🚚 Make onCreateNode API methods more readable

Name the fn in onCreateNode.js so it's more clear what it does → createMarkdownNodeFields
Return these functions in form of an object in our gatsby-node.js file instead of simply requiring the whole file.

🚚 Destructure gatsby-browser file to show fn names

Restructure gatsby-browser.js file to return the method names that we're using for each browser API endpoint so it's more clear to understand what each method does.

Other improvements

✅ Put back protocol check for killServiceWorker, which was only commented out for testing purposes
🔥 Removed unneded noscript fallback, which is now incl. in Gatsby by default since https://github.com/gatsbyjs/gatsby/pull/10945 (v2.0.90)
⬇️ Removed Bluebird dependency as we're only using Promise.all() which is ES6 native

TryGhost / docs