Handle thumbnail image along datapackage

[ridoo]

Currently, a default thumbnail is set. A user might put a thumbnail in the datapackage which could automatically be used as thumbnail. Usually, this is referenced by image from the datapackage.json.

Further aspects to consider with regard to thumbnails:

• tabular-collections get no thumbnail. The world extent is being used as thumbnail instead
• Thumbnail of a map is not updated when removing all non-tabular layers
• table icon should be moved and served via /static folder