redirect to default language; details of editions on a new editions page.

For the majority of books, there is only one language. In these cases, sending people to the editions overview is not necessary and just adds an extra click. Sending them from e.g. /user direct to the default book language and version, e.g. /user/en/latest is a better user experience.

This MR implements that workflow and moves the editions to e.g. /user/editions.

This follows readthedocs way of doing things...