Bring in Extractor project as a first-class module

[bctcvai]

Close down this repo:
https://github.com/cvisionai/thumbnail_extractor

but make a new module tator.util.extractor which is part of the unit test suite. Additionally, supply a workflow.yaml reference file for how to register it within a project.

If available, programmatic registration might be nice. (e.g. tator.util.extractor.register --project 3 --strategy-config <file>)