display the review and voting start dates in review lists

It'd be great to see the start dates in review lists.

Being able to sort them is especially helpful for reviewers, so they can see which of their assigned talks are up for review/voting at present.
The `group` property doesn't necessarily exist -- specifically it'll be `None` for anything that's not a talk (e.g. tutorials and posters).

Can you either confirm that this does the right thing in that case, or put this in an `if` statement?

(Maybe it makes sense to have these columns be contingent on `slug == "talks"` or something?)