Add archiving of closed issues

Adds an 'Archived closed issues' button to the top of the buckets index. Clicking it sets an `archived_at` on the closed `PrioritizedIssue`s in the user's current buckets and hides them. The buckets index only displays non-archived issues.

![archive-that-thing](https://cloud.githubusercontent.com/assets/849/3225131/ded24f9a-f048-11e3-88b3-3aa42e8f7f50.gif)

Fixes https://github.com/jonmagic/i-got-issues/issues/2.