Make it possible to login with external authentication for the root user

[Nicklas Nordborg]

Currently, login attempts to root account will never be passed on to external authentication. This was a safety measure to ensure that it is possible to login and make configuration changes in case there is a problem with the external authentication. However, it is also a weakness since the most powerful account is always only protected by a password. Since external authentication is probably used to increase security this should also apply to the root user.

In order to solve the issue with a non-working external authentication, an administrator should instead log in to the server (eg. via ssh) and then:

1. Stop the Tomcat server
2. Remove the JAR file responsible for external authentication from the plug-ins directory
3. Re-start the Tomcat server

[Nicklas Nordborg]

(In [6685]) Fixes #1907: Make it possible to login with external authentication for the root user

No special treatment for the root user when logging in.