Allow unprivileged user namespaces (fixes security features warning)

Allow unprivileged user namespaces in order to fix security features warning as implemented upstream by commit https://github.com/torproject/torbrowser-launcher/commit/692e28a11b8f99b752063287a537e62af98b87b9

Because the change is already implemented upstream, this patch can be removed with the next upstream version.

Fixes Debian Bug 1098845

See also Tor Browser issue 43101 and Firefox bug 1899516 for more information.

Should be applied to sid and trixie. Adds only a single configuration parameter userns to the AppArmor profile, might be considered a minor bugfix that should perhaps migrate to trixie sooner.

This merge request also fixes a lintian error: Homepage is defined in debian/control and does not belong into debian/upstream/metadata.