Change product_url to product_path.

I'm not sure why product_url was introduced, but I'm currently
encountering issues in production where it's not respecting subdomains,
HTTPS, and ports correctly.

Nearly everything else in Spree links to stuff using the _path helpers,
which I like a lot better.
Move price in to an offers block.

This is what the correct schema.org product implementation is.

Reference: http://schema.org/Offer
Merge pull request #3 from gmacdougall/2-3-dev

Simple fix for payment source.
Merge pull request #4 from gmacdougall/2-3-dev-url-never-path-forever

Change product_url to product_path.
Refactor Line Item Promotion Actions & Eligibility

Extract payload to a let statement.

Refactor eligible_rules to return rules.

This is helpful to understand which rules a promotion matched at some
point later down the promotion chain.

Extract line items to adjust in to Promotion.

This is part of moving the logic out of the promotion actions in to a
place that knows a little more about the context of what should be
applied.

Add actionable? call.

Move line item actionable to promotion.

No adjustments for non-actionable items.

The logic around this has changed a little, as adjustments are no longer
created for non-eligible promotions, due to the selection on actionable.

Remove products method (no longer used)

This is deprecated, as this logic is all done internally by the product
rule and related actions.

Closes #5036

Conflicts:
	core/CHANGELOG.md
	core/app/models/spree/promotion/actions/create_item_adjustments.rb
Set promo adjustment to 0 when not actionable

If an order qualifies for a line item promotion due to meeting some
condition (say, 10% off 5 or more), and the cart is updated so that the
line item promotion no longer qualifies, this will ensure that the line
item promotion is set to zero.

There is additionally some changes to hide promotions which are zero, as
they're not super interesting. The promotions are zero-d out and hidden
so that if at some point in the future the order re-qualifies, the
amount will be set correctly.

Closes #5245

Conflicts:
	frontend/app/views/spree/orders/_form.html.erb
Merge pull request #5 from gmacdougall/2-3-promotion-line-item-eligible

2 3 promotion line item eligible
Default order_factory line_items_count to 1

This took 40 seconds off of the "core" specs on my computer (3min30s -> 2min50s).  I didn't measure the other suites.
Given the speedup it seems like a reasonable thing to do for the cases where you only really need a single item.

Closes #5085

Conflicts:
	core/lib/spree/testing_support/factories/order_factory.rb