Callback URL registered incorrectly if using route groups

[0xTim]

When registering callback URLs with Imperial using router.oAuth, if the router is registered on a route group (e.g. /api) then it registers the callback url on the application incorrectly (such as under /api/api)

[fpseverino]

This issue isn't really solved for most services (10 out of 13), because those services need in the callback body a redirect_uri. We get it from the callback argument of RoutesBuilder.oAuth, so it needs to be a full URL (e.g. http://localhost:8080/api/oauth/service), but we also use callback to create the route with the RoutesBuilder, so if the latter is grouped let's say on "api", the resulting route is going to be /api/api/oauth/service.