Bring over much of the work from the other branch, but do it the other way: Special handling in CatalogController.
I think this is the approach we want on overrides. Fixes #572. Fixes #573. Fixes #568.
More override content. Addresses #596, but there are some other credit pages I want to clarify before closing.
Merge pull request #605 from afred/override-572

Use /view/overrides for collections.