Store download state and pack selection in database

[dbnicholson]

Currently we use a combination of web cookies and heuristics to decide what the current download state is. That makes it tricky to figure out if the downloader needs to run or not (see #863). I think it would be better to store the download state and pack selection in the database. That would mean the current state could be reliably queried, and it would be strongly persisted (unlike cookies, which can be cleared).