Deleted the continue button on the RMA index

Deleted the continue button on the RMA index.
It does not have any function there.
Fixes #4891
Lock jquery-rails to 3.1.0 as 3.1.1 breaks data-confirm. Fixes #4892
remove 2-3-stable gem dependencies

since spree got bumped to 2.4.0.beta

Fixes #4893
make incl/excl in ShippingRate#display_price translatable

Fixes #4894
make click_and_drag_on_the_products_to_sort_them in spree/admin/taxons/index translatable

Fixes #4894
New paradigm for customer refunds

Initial step in a larger piece of returns & exchanges improvements.

Major

* new "refunded" state for return authorizations
* separation of refunds from payments
  * add a new refund model
  * use "refunds" in place of negative payments
  * stop generating order adjustments when receiving return authorizations
      * we no longer need to create order adjustments in order to provide refunds
* updated admin ui
  * added a "refund" button in the return authorization flow
  * list refunds along with payments
  * add payment identifier column to payments list

Minor

* extra validation on `ReturnAuthorization#force_positive_amount`
* improve some existing specs by using real `ActiveMerchant::Billing::Response` objects instead of doubles

Generated by @jordan-brough & @richardnuno

Fixes #4883
Take adjustments into consideration when calculating RMA totals

* Added #total_per_item to line_item that calculates total per item including adjustments
* Added new column in the RMA admin screen to display the total per item for a variant
* Updated javascript that suggests the RMA total to use the total per item when calculating

Fixes #4895
Only destroy tax adjustment for open order when the associated tax rate is deleted. For closed order just nullify the source_id.

Update line_item & order when adjustments are deleted.

Add rspec tests to cover cases where adjustments get (or not get) destroyed when taxrate/promotion is deleted.

Move deals_with_adjustments to a separate mixin.

Add new mixin to core

Revert previous changes to remove unnecessary tests.

Remove order.update! and simplify rspec.

Fixes #4828
Fixes #4771

Conflicts:
	core/spec/models/spree/tax_rate_spec.rb