Modularise scripts

Created by: djgrant

Following the discussion and for reasons mentioned in #1431 (closed), this PR aims to refactor the webpack abstractions found in the start script out into their own utility modules.

I've aimed to maintain the general narrative of the script by keeping the running of methods in the script and using callback to eliminate interweaving.

Summary of changes

/config
  + webpackDevServer.config.js
/scripts
  /utils
     + addWebpackMiddleware.js
     ~ createJestConfig.js
     + createWebpackCompiler.js
  ~ eject.js
  ~ start.js 

• All files found in specified folders get copied to the app directory on eject
• Except files flagged with a // @remove-file-on-eject comment, which will be skipped i.e. scripts/eject.js & scripts/utils/createJestConfig.js