Fixes bug 913039 - Added a cron job to delete old elasticsearch indices.

@phrawzty I know you wanted to work on this, but as I already had a good chunk of the code started I thought I could just finish it. 

I *think* this covers all our cases. We have 3 kinds of indices at the moment: the expected ``socorro%Y%W`` ones, then we have those that were reindexed and look like ``socorro%Y%W_%Y%M%d``, and then we have one special index called ``socorro_emails``. I don't think there's anything else, and the unit tests should cover all those cases. 

So, r? @phrawzty on the logic. @peterbe, please take a quick look at the code quality if you'd like.