Implement debian:qa-results collection category

For regression tracking (#791), the blueprint involves a new debian:qa-results collection category. Before we can do much else on that project, we'll need to implement that new collection.

The collection manager needs to support debian:autopkgtest, debian:blhc, and debian:lintian artifacts as described in the blueprint, as well as debian:qa-result bare data items; that bare data category doesn't exist yet and will also need to be modelled. The manager should construct the appropriate collection item names when adding items.