Change Foreign Key relationship for ResourceBase and Thumbnail

__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 for some reason.

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.