Book home pages

Created by: seanmadsen

Commits in this PR are based on my refactor in #45, so please merge #45 first.

This PR adds the ability to view a "book home page", that is one page in the Symfony app that shows all the details of the book, namely the different editions available. This way, if a visitor goes to https://docs.civicrm.org/user they'll see all the available editions of the User Guide. Also, /user/en redirects to /user.

Preview: