Archiving tweaks

Tracker: https://www.pivotaltracker.com/story/show/60500360

Various tweaks to the archiving interface and front-end, as described in the tracker story:

* [x] The word "[Archived]" needs to be added to the meta title
* [x] When archiving, the public explanation should be mandatory
* [x] When choosing "published in error", if "redirect to URL automatically" is checked, the public explanation field should disappear
* [x] Mandatory fields have a red asterisk
* [x] Show validation warnings on unpublish form, rather than edition page
* [x] Fix archive notice styling on collections
* [x] Fatality notices archive notice to not say "field of operation"
* [x] Show archive notice on detailed guides