Send import notification always for re-orgs

This pr changes the behavior of sending import notifications. Before we
only send notifications when importing blocks on the tip of the chain or
on similar conditions. However we did not send a notification when we
for example being in a state where we import multiple blocks to catch
up. If we re-org in this process, systems like the transaction pool
would not be notified about this re-org. This means, that we would also
not resubmit transactions of these retracted blocks. This pr fixes this,
by always sending a notification on a re-org.

See
https://github.com/substrate-developer-hub/substrate-node-template/issues/82
for some context about the bug.