Prevent adding items without stock via admin UI [Fixes #3041]

If stock item on hand is 0 and it doesn't allow backordering admin
shouldn't be able to add the product

[Fixes #2998]
Taxon edit form layout tidyup

Fixes #3042
Consolidate spree/order attributes for coupon_code
Add backorderable default config to StockLocation [Fixes #3045]

Stock items backorderable values will be set according to this config
every time either a variant or a stock location is created
Bump caypbara to 2.1.0:

* Add timeout into config/database.yml template to avoid SQLite3 DB locked out
* Use lenient non-exact setting for Capybara element matcher (same as 1.x)
Capybara 2 updates.