fix font-awesome icons for admin taxons tree

Closes #4717

Conflicts:
	backend/app/assets/stylesheets/spree/backend/plugins/_jstree.scss
Dont mess unrelated records on CreateItemAdjustments#destroy
fix some specs

these specs were broken by f1658936aa021a9cded44d4df59eab0c884ac06e in 2-2-stable
Add instructions for contributing to Bonobos fork
Update orders.promo_total in OrderUpdater

NOTE: Cherry-picked from this spree pull request until it gets officially resolved: https://github.com/spree/spree/pull/4607

orders.promo_total field wasn't getting updated in certain cases, like when you add the promo first and then a line item afterward.
the *adjustment* got updated and the correct number was displayed in the cart, but the orders.promo_total value was incorrect.
Ensure correct payment amounts

When paying with only one payment method, ensure that the amount charged equals the amount due.
Pass in order params on order creation so they can be overridden
fix for payments made via /admin

this is the immediate term fix while we address several issues around this.
this was causing double-billing on orders placed via /admin.
Use an InMemoryStore to access certain Spree preferences.