Switch to storing sessions in DB

* We were exceeding the cookie size limit in our sessions
(save_mapping_ids)
* Send less data with each request
* Table generated using `rake db:sessions:create`
* See
http://stackoverflow.com/questions/11706297/rails-3-storing-session-in-a
ctive-record-not-cookie
Merge pull request #194 from alphagov/store-sessions-in-db

Switch to storing sessions in database