[Bug 914266] Archived questions

This is a few things

* Adds `is_archived` to Questions models
* Adds `question_is_archived` to the ES indexes.
* Add an archived filter to advanced search.
* Updates the auto-lock cron job to be an auto-archive job.
* Adds the first and second South migrations.
  * Schema migration to add the column.
  * Data migration to change the all locked questions to archived questions.
* Does the two step ES dance.
* Updates UI stuff to treat archived questions like locked questions, and adds a note.

Very useful commands:
* Run the migrations from this PR: `./manage.py migrate questions 0003`
* Undo all the migrations: `./manage.py migrate questions 0001`

The bug mentions changing default search to deal with archived questions like it does locked questions. Turns out that default search doesn't actually filter on locked questions (as far as I can find). It is simple filtering by a particular age. We might want to change this.

This took a bit longer than I feel it should have, partially because I was messing around with South a bit. It has more fiddly bits than Schematic, but the forward/backwards support is super nice. Overall it seems like a much more robust migration system.

r?