Task: thumbnail for tabular data

[mwallschlaeger]

currently nobody really looked on how the thumbnails of tabular data are looking or can be configured in the overview list of geonode.

[ridoo]

Currently, there is a table-icon.jpg shipped with the importer_datapackage module. However, this was more like testing things out and was not saved as thumbnail. With regard to #6 (here: avoid automatic thumbnail creation via WMS) I take this up again in context of this issue as once a thumbnail is set, no WMS will be requested for an image.

TODOs:

• [x] Set the table-icon.jpg by default for non-tabular datasets
• [x] Remove the UI option to set a thumbnail based on WMS
• [ ] (optional) let the user upload a thumbnail alongside the actual data (see this issue #21)