Export Collections as Bags

[ntallman]

As a digital preservation librarian, I would like to export entire collection and its works in a single bag, So that I can send them to long-term preservation storage.

Acceptance criteria:

• works can be exported as LOC Bags
• works can be exported as APTrust Bags
  • 5 TB bag-size limit, if over bags need to be split
• export is triggered by GUI and transferred to pre-determined store (TBD)
• preservation masters and text files are included in the bag
• service masters are included in the bag if they were pre-supplied, but not if they were CHO generated
• all metadata is included as RDF

Related: #441

[ntallman]

Some of the acceptance criteria may be out of scope for CHO, but keeping this in to explore in future.