Use general preferredName / variantName properties

[acka47]

From #1:

We don't want specific name properties like preferredNameForThePlaceOrGeographicName and variantNameForThePlaceOrGeographicName. For all entities, we should just use preferredName and variantName.

This will allow us to query the whole data in a uniform way. (The type is made clear by other means so that we don't need the specific properties.)

[fsteeg]

Deployed to test: http://test.lobid.org/authorities/4074335-4.json

(@ChristophEwertowski: We have no production system yet, so just +1 and close if this looks good.)

[ChristophEwertowski]

+1