Fix authors field and add publishers to documents

This PR is necessary to publish successfully with the latest cnx-publishing.

Note: "username" is no longer a required field in the user json (used in submitter and authors)

***

- Include all expected fields in the user schemata
```
The expected fields are "id", "email" (optional), "firstname"
(optional), "surname" (optional), "type" (optional, defaults to cnx-id)

User "type" needs to be set to "cnx-id" for publishing to recognize the
user

Add extra validation to "authors" in document to make sure there is at
least 1 author
```
- Add publishers to documents
```
Publishers is now required by cnx-publishing.

MANUAL-UPGRADE: Add a json column "publishers" to the document table in
postgres db
```