Do not show login-button but the username if the remote user variable is set

Whenever the VaadinRequest.getRemoteUser() function returns a name, we hide the login button. We also show the user their current user name.

This approach is useful in situation where you have a authentication infrastructure like Schibboleth to restrict the access to the ANNIS web frontend but have a "no security" service in the background. It also partly solves #200