Set a default key for Ideas as with Threads.

DRY? We've heard of it. If only we had role-based OO:

https://github.com/duckduckgo/community-platform/blob/6a566dee/lib/DDGC/DB/Result/Thread.pm#L154-L162
Merge pull request #226 from jbarrett/ideas-det-default-key

Set a default key for Ideas as with Threads.