Reverse ForeignKey relationship between Thumbnail and ResourceBase

__What does this PR do?__
Reverses ForeignKey relationship between Thumbnail and ResourceBase to be Many -> One.  Should solve repeated thumbnail issue on Updatelayers execution.  The result is a single file on the filesystem per ResourceBase object. Cascade deletes Thumbnails on ResourceBase delete.  Doesn't delete thumbnail file on disk, will see about adding.  Please test as this is a bit untested.

Note: Also fixes a legacy issue with Updatelayers code in gs_slurp where some code was overwritten in a bulk merge following 2.0 release.