Record results of lookups somewhere for web UI

Some web UI views (currently just for autopkgtest and lintian, but there's no particular reason the requirement should be limited to just those) render links to artifacts used by work requests. With the move to the new lookup syntax, they need to repeat the lookup in order to figure out which artifacts to link to. However, the current implementation means that the artifacts found when the task itself performed the lookup (assuming it's been run) may not be the same as the artifacts that the view finds, because the collection may have been changed since then.

I think that:

• the task needs to communicate the results of its lookups back to the server
• the server needs to store those somewhere in the database
• views need to use those stored results