Add legacy MD5 Minter

Submitting this for discussion.

Interested in ideas about more robust testing of edge cases for existing ID's. This needs to be able to replicate https://github.com/dpla/ingestion/blob/develop/lib/akamod/select-id.py#L55
It's pretty important to get the legacy identifiers right.

I'm thinking the UTF-8 issue may be best handled in the Primo harvester, while I'm not sure what the best solution for the underscore issue is.

Do Primo ID's actually have any spaces?