[Issue995] Prettify bug set view, update migrations

Edited: See https://openhatch.org/bugs/file685/bugset-fix.png for preview.
`bug.skills.count` makes sense to me.

You're probably right that `if bug.skills` doesn't do anything reasonable.

I don't really like `first` but I won't object to you using it, since Django provides it (or at least, does in the future!).