Re-choose best available promotion when an order changes

Before this change, we could end up with multiple eligible promotions on
the same order. This happened because changing the order (for example,
by adding line items) rechecked the eligibility of promotions,
potentially making multiple ones eligible, but forgot to re-choose the
one best promotion.

This change adds a call to the logic that chooses the best promotion any
time the ```OrderUpdater``` recalculates adjustments.

Adds test to ensure we do not apply multiple line item promotions

Spec and style improvements

Runs two specs twice with different ordering to ensure the order in
which promos are applied doesn't matter. Also cleans up some style
issues and removes an unnecessary stub.

Add specs for taxable items + promos

Correctly limit a promotion to its maximum amount

You should never be able to reduce the price of an adjustable to less
than zero. Thus, we have to guard against a promo applying a discount
larger than the pre-promo amount.

However, as we also do recalculations, we cannot just use the
post-adjustment amount as a limit here. We have to use the
pre-adjustment amount -- i.e. pre-tax, pre-promotions. That is correct
because a) tax gets calculated on the post-promotion amount and b) there
can only be one active promo per line item.

fix for line_item amount instead of price with associated spec

Conflicts:
	core/spec/models/spree/promotion_handler/coupon_spec.rb

CreateItemAdjustments handles only LineItems

Fixes #4675
Fixes #4775
Fixes #4799