Change: Rename previews to thumbnails

Until now, we have been referring to preview images in OcFilesTable and OcResource as `preview`.
The technically more correct term for our use case would be `thumbnail` and since we're introducing 
proper previews in the web repo now this change renames `previews` in ODS to `thumbnails` to avoid 
naming conflicts down the road.

https://github.com/owncloud/owncloud-design-system/pull/1429
