Add statistics to admin home page

[ecumike]

Add some metrics and data auditing to admin home page, ex:

• Avg # of runs per URL?
• Latest test run URL and timestamp.
• # of URLs that have NO valid runs (this surfaces bad URLs).
• # of URLs that have NO runs (shows URLs in the system but haven't beed tested yet).
• Latest URL and timestamp that was added to the app.
• Throughput of queue and progress? Need to check with @daviddahl .. I think this web service was setup already. May not be committed yet though. TBD.
• Etc.