Move API endpoints into a dedicated namespace

This MR moves the API endpoints into a dedicated api/v1/ namespace, which will ease the documentation and maintainance of the API, as well as easing hosting behind a reverse proxy.

Indeed, until now, every request had to go through the reverse proxy. With this commit, one can easily pass api/* requests through the proxy, while serving all the rest of the content directly through the webserver, using build/client as document root. This should improve a bit the performances of such a setup.

Finally, it is worth mentionning that Kresus can still be directly served using Node as it used to be, the behavior remains exactly the same.

Any review and feedback is greatly appreciated :) Please note that this MR builds on top of !429 (merged) and should then be merged after !429 (merged) has been merged.