Always pass a civcode to Viewer when instatiated by the session gui

Previously this was only done for technologies.

The Viewer tries to determine an appropriate civcode from a passed template,
however trainable gaia-animals have a template-assigned civ of gaia (and
technologies don't have one at all).

The behaviour of the Viewer isn't changed by this commit: it will still prefer
whatever civ is set within a template before falling back to the passed civcode.

However, trained gaia-animals now have something to fall back to, permitting the
appropriate build list (the list of buildings that train the animal) to be
displayed.