Don't modify Apache configuration file supplied by FreedomBox

It appears that when setting up default application for FreedomBox, we are modifying the file /etc/apache2/conf-available/freedombox.conf. This will result in a configuration file prompt while upgrading freedombox and will also lead to unattended-upgrades refusing to upgrade freedombox.

Instead write to another file that will override the existing configuration option due to how RedirectMatch rules are ordered by apache.