Use global instead of per-method test dates.

Required several schema changes:
* Add num_applications to offer model to track number of times an offer
  has been applied within an order.
* Add max_global_applications to offer model to set a limit on how many times an offer
  can be used.
* Add frequency to order discount to provide insight into how many times
  an offer was applied for a particular order.

Structure of dashboard changes to talk about offer 'availability'
instead of assuming a start- and end-date.

Also added tests for vouchers

Plus tweaks to test method names

Also split them into unit and functional tests.