Update base_form.volt

If style of header is not set in form xml, then PHP Notice appeared in log:
PHP Notice: Undefined index: style in ......

lagg interfaces, restructure interface_lagg_configure() and add mtu option, for https://github.com/opnsense/core/issues/1709

legacy_interfaces_details() parse issue on fetching laggports, needed for https://github.com/opnsense/core/issues/1709

Networking, side affects. when an "other type" interface is configured it's automatically created, but if the interface got unconfigured it's dropped,leading to broken laggs for example.

We should trust the "other type" create/drop to handle the interface itself and interface_bring_down() should only unconfigure it, so possible parent/child interfaces won't break for no reason.
Fixes part of https://github.com/opnsense/core/issues/1709

We are forcing a change through the installer, which is where
the majority starts from.  We don't want to enforce a change
twice.

We can't really see if we installed or went through a factory
reset, so this is (was) the best of both worlds in conjunction
with the new installer feature.

While here, annotate that it is possible to leave it "empty".

This reverts commit 5eb2c986.

PR: https://forum.opnsense.org/index.php?topic=5466.0

PR: https://github.com/opnsense/core/issues/1714

opnsense-update can still be used to selectively upgrade, but
here we shouldn't take any chances.  Worst case the system will
reboot prompting "nothing to do", which isn't true because we
only know about the base and kernel, not the packages state.

We want to pull changelogs also when we look for upgrade instructions,
in order to safely show then afterwards if we can indeed upgrade from
there.