Requests can only belong to one batch

This has been enforced at the database level for
a while. If we enforce it at the rails level we
should get cleaner handling of mistaken double
batching, and can simplify some of the code.
Merge branch 'next_release' of github.com:sanger/sequencescape into requests_belong_to_one_batch
Fix batch_request_typo
Properly handle batchless requests
Restore create_batch_request! method
Merge pull request #868 from JamesGlover/requests_belong_to_one_batch

Requests belong to one batch