Add "Browse this project in Overview" link

We'd like DocumentCloud to link to the import page on Overview. I've added DocumentCloud-side code to do this.

There are some prerequisites, though ... in order:

1. documentcloud/documentcloud#57 needs to be solved
2. documentcloud/documentcloud#25 needs to be solved
3. DocumentCloud needs to be deployed
4. overview/overview-server#473 needs to be solved
5. Overview needs to be deployed
6. Finally, this pull request can be applied