Support unauthenticated submissions

[ahayes]

There is a need to support the optional ability for unauthenticated submissions to the atlas when the submission database functionality is enabled. This will permit atlas patrons without user accounts to propose new documents to be reviewed by atlas vetters similar to existing authenticated submissions.

In the future it would be nice to also be able to have unauthenticated patrons propose edits to existing documents, deletions of documents, and hopefully even auto approval for public and public_* layers in the future as has been possible without an enabled submission database (see interrupted Issue #1085).