Make mw.wikibase.getEntityObject() actually return non-Legacy data

Also fixed the module to no longer guard against this, so that
the integration tests will catch this in the future.
On top of that, we now disallow the creation of mw.wikibase.entity
objects with legacy / invalid data.

Change-Id: Ie52d887b9a2dda98187cc23ce400614ca1bf2fac
Merge "Make mw.wikibase.getEntityObject() actually return non-Legacy data"