Add Spree::Admin::RootController for redirecting purposes.

Currently, is really difficult to redirect to a different tab when the
user has logged in if said users cannot at least :admin, :index
`Spree::Order`s.

This commit adds a controller that handles `/admin` route so anyone can
easily redirect to another page (or implement any other logic).
create proposed shipments if is no shippment data
also make sure to set the cost provided after order update

Conflicts:
	core/lib/spree/core/importer/order.rb
	core/spec/lib/spree/core/importer/order_spec.rb
Looking up stock location by the admin name as well

fixes #5484
Add spec for looking up stock location by admin name.
Remove spec invalidated by https://github.com/spree/spree/commit/a13524a6cfd363c441ff1607a584a3be07e1124f now creating shipments.
Do not allow leading or trailing dot(.) in the first part of email address

Add rspec for email validator

Fixes #5483
Simplify set_current_order. Fixes #5382
Search for guest order first before running Order.new

Fixes #5441
Add inverse_of for more associations involved in cart processing
Fixes #5488
Loosen kaminari dep. Fixes #5486
Fix guides links.
rspec-retry is just a test dependency.
remove extra dependent destroy
fixes #5492
Update hub links to wombat
Fixes messaging after applying coupon code promo with CreateLineItems action. Fixes #4697

Conflicts:
	core/lib/spree/testing_support/factories/order_factory.rb
	core/lib/spree/testing_support/factories/promotion_factory.rb
	core/spec/models/spree/promotion_handler/coupon_spec.rb
Refactor Spree::ProductHelper#variant_price_diff

* Change the logic of the code to make it easier for the next
  commit.
* Remove duplicate code from the HTML generation.

Change #variant_price_diff to handle when the product amount is nil

Fixes #5493
Allow order admin search via sku

remove white space

switch from expect to allow

fixes #5494
Add required attributes to required product admin form fields

* Adjust spec example inputs to allow submit with that restrictions.

fixes #5496
make bogus gateway match actual gateways. ie. don't assign profile id for failed payments

fix handling of retry payments and multiple payments

fixes #5499
Fix displaying product & taxon names when meta_title present.

Fixes #5502
Fixes #5501