Only add 'https' when preview page in production
Add a .env.dev file and added instructions about it in the README
Do not set qa url when I have FRONTEND_URL defined
Add http to preview page link when is in dev mode

* Change the ENV variable, removing the 'http://' prefix.
* This is more consistency with the QA and Prod urls
Merge pull request #101 from moneyadviceservice/frontend-preview

Frontend preview (improvements)