fix bug 1112676 - show deleted docs on rev dashboard

Show "deleted" marker on the revisions dashboard when the document has been deleted. To spot check:

* [ ] Compile stylus
* [ ] Edit a wiki page
* [ ] Delete the wiki page
* [ ] Check the [revision dashboards](https://developer-local.allizom.org/en-US/dashboards/revisions)
* [ ] Should see "deleted" on the row

![mdn_rev_dashboard_deleted](https://cloud.githubusercontent.com/assets/71928/5511379/6a71491c-8792-11e4-8ebb-d1a8da4f5cfe.png)

#f00