Add extern authentication

Provide a new settings authenticationMode and delegate the
authentication handling to the webserver when the external authentication mode
is set. Add a new view 'logout' that will be shown after logout from external
authentication as the regular redirect to login is not possible.

refs #5405
When webserver setting is missing regular authentication will be used

Check if the remote user is set and use regular authentication when the key is missing.

refs #5405
Merge branch 'feature/accept-web-server-authentication-5405'