Closes #30.

Closes #30.

Which is more pythonic.

The possibility to register all routes within a route prefix allow to remove
this complex code.

Also remove unecessary check on openapi 'paths' property presence.

The base url is already here for this.

If a "json" renderer is already defined in the project, a conflict appeared.
Now the renderer is prefixed, there is not more conflict.

Reverts to the base pyramid_jwt implementation but with a custom serializer which simply returns the given value. Needed as the JWT token is already base64 encoded.
This fixes authentication issues using the cubicweb_web login forms.