Fix login redirect for reverse proxy setups
In case of ISPConfig being set up with a reverse proxy under a sub-directory, the current redirect will lead to an unknown invalid location. Example: ISPConfig is configured via reverse proxy under https://myserver.com/ispconfig/ - the login will redirect to https://myserver.com/login/ instead of https://myserver.com/ispconfig/login/ I did this change manually several times now after every update of ISPConfig but would like to see this in the default setup. The relative redirect target should not have any negative effect on standard installations.
Should fix #4378