Switch to using Mongoid

MongoMapper doesn't deal well with mongo re-elections, and raises errors.  Mongoid handles this well.

Since Mongoid doesn't support Rails 4 yet, this has required downgrading to Rails 3.2.


Note: https://github.gds/gds/alphagov-deployment/pull/354 needs to be merged at the same time as this, or the preview deploy will fail.