Finalize the internal data model and decide upon semantic choices for clarity between Commons items and Wikidata items ("labels" vs. "captions", etc.)

The internal MediaInfo data model needs a few tweaks. MediaInfo should use “caption” instead of “label” (which Wikidata uses). They are very similar, but we want to make the distinction for semantic clarity.

For the same reason, "description" in MediaInfo should be changed to "summary".

If other conflicts/confusion between naming conventions in MediaInfo and Wikibase become apparent, address those as well.