Include uploaded file in paver reset
Simplified handling of thumbnails.

* Added tests for thumbnails in Layers and Maps.
* Removed ThumbnailMixin class and just created methods in ResourceBase.
* Removed need to call save again in catalogue or mess with signals.
* Added logic to return the link to the WMS image in thumbnail if WMS is not yet available during upload.
* Uploads are now around 2x faster.