Re-work asset stats endpoint

This PR re-works the stats endpoint as outlined in #84 (closed) and #85 (closed). Commit a64b5003 merely tidies up the existing test a little by directly creating the Asset entries in the database rather than having the boilerplate of going through the API. Commit e9081e65 addresses #84 (closed) by excluding deleted assets from the returned stats. Commit abc01933 builds on this work and re-arranges the results from the stats endpoint to have the grouping outlined in #85 (closed).

Closes #84 (closed). Closes #85 (closed).