Apache: prevent Referrer leaks via WebUI

Apache: prevent Referrer leaks via WebUI

By default, even modern browsers sent the URL of ther originating
site to another one when accessing hyperlinks. This is an information
leak and may expose internal details (such as FQDN or IP address)
of an IPFire installation to a third party.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>