JSON routes do not need it but added for consistency

The login route is now disabled by default.
It was previously enabled by default.
It is why it is a breaking change.

Mount on "/" when we are on an older version of cubicweb missing the receives-base-url-path OR when the receivese-base-url-path option is set to False. This option has been added in version 4.2.

Otherwise we are serving our routes on the full base url.

Closes #34.