Add more provenance information to imported data

[mikesname]

The maintenanceEvent type already exists (and is used by repositories) for this purpose, so we can add maintenanceEvents for things like:

• data from USHMM
• converted by EHRI

This will then be combined (on the frontend) with the online history, which includes such data such as:

• imported by Ben
• annotated by Mike

On the backend side we need to capture pre-import history with maintenanceEvents.

[bencomp]

Duplicate of #33 ?