[WIP]Encode default globe in family file.
The current default assumes that every Wikibase installation has a
globe named 'earth' which need not be the case. It also assigns
this globe even if overridden by globe_item.
Instead encode the default globe in the family file and only fall
back on the default if neither globe nor globe_item has been set.
Also clarifies that globe_item takes precedence by mentioning this
in docstring and using entity instead of globe in representation.
Note that 'earth' is still implicitly implied in