Date in the project download tables is when the release node was edited, not file timestamp

Splitting this out from #539682-8: Replace hard-coded release download table with a view so it's more visible...

The datestamp in the release download tables on project nodes is currently using the node edit time for the release nodes. It really wants to be the max file timestamp for all files attached to each release node. I think I need to write another little custom field handler for this. Stay tuned...