- If start_datetime is null, then the offer has no start date.
- If end_datetime is null, then the offer never expires.

Use the ActiveOfferManager in Applicator.get_site_offers instead of
reimplementing the same logic there.

Fixes #2344.

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

Oddly enough, the warnings were only thrown under Python 2.6. Should
investigate!

This required extending the offer factories to support setting the
start- and end-dates (looking forward to using FactoryBoy now).  Some
additional tests were added for the active manager.

Fixes #1128