firewall: Make default zone as 'external'
firewall: Make default zone as 'external'

Set the default firewall zone.  When network connections are configured
outside of FreedomBox/Plinth, they will not be able to serve the Plinth
web interface.  This is because all such interfaces will fall in the
default firewall zone and that is, by default, 'public'.  On 'public'
zone we don't allow Plinth web interface as this zone is not managed.

Configuration of network connections happen outside for
FreedomBox/Plinth for various reasons:

 - Existing network connections before installation of freedombox-setup

 - Connections configured in /etc/network/interfaces

 - Connections manually configured using nmtui

 - Connections created using GUI environments such as GNOME

Rather then clearing out /etc/network/interfaces during setup and
expecting the connections not to be created outside of Plinth, setting
the default firewall zone is a better approach.  This default zone
selection fits with the main purpose of FreedomBox to be a router which
is also reflected by the fact that only 'external'

freedomboxtwh/Plinth,harry-7/Plinth,jvalleroy/plinth-debian,vignanl/Plinth,jvalleroy/plinth-debian,kkampardi/Plinth,harry-7/Plinth,vignanl/Plinth,kkampardi/Plinth,kkampardi/Plinth,vignanl/Plinth,kkampardi/Plinth,jvalleroy/plinth-debian,freedomboxtwh/Plinth,harry-7/Plinth,freedomboxtwh/Plinth,jvalleroy/plinth-debian,vignanl/Plinth,harry-7/Plinth,freedomboxtwh/Plinth,jvalleroy/plinth-debian,kkampardi/Plinth,freedomboxtwh/Plinth,harry-7/Plinth,vignanl/Plinth