Implement the API thumbnail endpoint method to delete the thumbnail for a resource

[giohappy]

A new action delete_thumbnail for deleting the thumbnail for a resource will be added to the ResourceViewSet APIs

• The action implements the same access permissions as the other thumbnail APIs
• The action accepts POST (we don't use DELETE since we don't have a /thumbnails endpoint where the CRUS semantics would make sense)
• The thumbnail's image is removed from the disk and Resource.thumbnail_url is emptied

[Gpetrak]

@giohappy I guess we have to clear also the Resource.thumbnail_path right ?

[Gpetrak]

@giohappy the PR for this issue is available here: #13032