Only show a preview of the batch

Showing the whole of very large batches would cause very high view rendering
times and hence timeout, plus it would have been unusable.
Process small batches inline

This makes much more sense when adding (possibly updating) a single mapping or
a few mappings, which is definitely the normal case.

This reinstates the modal confirmation dialogue for small batches.

The specs for mappings_batch_outcome_presenter.rb were cribbed from the specs
for BulkAdder.

Reverted features changed earlier in this branch which run small batches to
look for the modal, rather than the "x of y processed" flash message used for
large batches.

20 was chosen as the upper limit for a "small" batch because it should be easily
processable within the 15 second timeout we have for admin apps, but is just
above the average batch size according to Google Analytics.