Complete rewrite for v2

[callumlocke]

Tentative plan:

• Make a new repo at financial-times/ig-onward-journey (and add deprecation notice and link to old repo's readme)
• Write a readme that states what the app is for, and lists the endpoints with examples and clear explanations of how they will be consumed
• Set up the new repo to auto-deploy to ft-ig-onward-journey on Heroku, with PR review apps
  • Ideally also rename the old app to ft-ig-onward-journey-legacy and update the Apache config on the IG server accordingly
• Make a new entry in the Next service registry for /__ig-onward-journey
• The new app's routes all start with /v2, and it uses CAPI v2 (see Add support for CAPI v2 #24)
  • Do not bother writing backwards-compatible /v1 routes – the old Heroku app will continue to serve these for old clientside apps still using it
• Probably use micro to write the app, and probably avoid Babel (just use Node 8 for async functions)
• Write tests for each endpoint, and get them running on CircleCI