Add association between batches and public bodies.
Split the creation of a batch and the associated requests.

We're going to want to actually create and send the requests later.
Use public bodies not requests in find_existing

The requests may not have been created at this point.
Refactor specs to make adding batch changes easier.
Display batch requests for user on 'my requests' page

This is the most rudimentary possible way to give them access to the batch request urls, pending #1239
Simplify return value, set sent_at in create_batch!
Update notices for offline sending.
Show public body list when batch has not been sent.
Add a mailer method for sending a batch request notification
Send batch requests as a cron job.