Frontend preview (improvements)

I open this PR because I was trying to test the links to the pages works as intended when you preview the article, and I follow all the confluence steps, and all the steps could be improved, in order to speed up our development time. That said I made some improvements:

* Added the .env.dev and put instructions in the Readme.
* Fixed the bug precedence when you have defined the env variable.
* Move this logic to a presenter (Still it's not a good place to have some rules, but for now it's ok).

Other purpose to open this is to open a discussion about this env vars. The dotenv-rails in in our Gemfile, but it should ne on the test and development group?
Consistency would be to have the url scheme here and expect only localhost:5000 in config.preview_domain.