Include all vendors in dashboard stats

[aslagle]

The vendors in the dashboard is currently a hard-coded list: https://github.com/NYPL-Simplified/circulation/blob/master/api/admin/controller.py#L1385

It should include all collections, including ones where the data source name is specified by the admin setting up the collection.

[jonathangreen]

+1 on this one

I had a tester asking why their books weren't importing since they weren't in the list.