Set a default key for Ideas as with Threads.

Fully non-ASCII and non-alphanum idea titles can result in keys not being generated which means a method can never be matched to present the Idea content - idea_redirect continues to redirect to itself without a key.

Example:

https://duck.co/ideas/idea/283/

Thanks to @mwmiller for report.

Also needs:

```sql
UPDATE idea SET key='url' WHERE key='';
```