Publish documents

Publish documents by POSTing to /publish:
```
{
  "submitlog": "New version!",
  "items": ["c58da964-f96a-4dcb-be1c-f0f982bdea0e", "3dc464d3-c94d-4949-a327-f685181c5179"]
}
```

GET /contents/c58da964-f96a-4dcb-be1c-f0f982bdea0e will return something like:
```
...
"state": "Processing",
"publication": "143"
}
```

Publishing binders doesn't work yet.