- allow creation of offers of all types
- allow creation of vouchers with multiple offers
- allow creation of voucher sets of all usages, and with multiple offers

As in `test_model.py` -> `test_models.py`

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

* This gives more flexibility to offers in how they are used.
* Am using a jQuery datetimepicker from: http://trentrichardson.com/examples/timepicker/

Following guidance from Tangent's UX team (blame them!)

* Split 'metadata' step into 'name and description' and 'restrictions'
* Add progress bar
* Add summary of entered data in right-hand sidebar
* Clean up breadcrumbs and URLs

Still needs from FED love.