Package PurlDB as its own library, extract MatchCode to own repo

It would simply vastly deployment and configuration to have this improved setup:
- [ ] Create a library of PurlDB available as a PyPI package
- [ ] Extract MatchCode (and possibly the MatchCode toolkit) in its own repo

The new matchcode app will have its own (simplified?) settings based on manually merged settings from SCIO and PurlDB, and will require SCIO and PurlDB as libraries.