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).
Start release notes.
Some updates to release note links.
Update basic RMA guide.
Remove extendable line items from notes.
Fix example to contain expectation
Fixes #5449
allow guests to use the payments api for their orders
Fixes #5450
Improve localization of stock_locations/_form
Fixes #5454
Fix factories to generate unique SKUs

* Choosing random SKUs even from a high range can yield duplicates
  resulting in build failures.
* Kernel.rand(9999) was definitively never a good choice due the
  birthday paradoxon lets manifest a duplicate quite often
* Uses sequencially generated SKUs defined from a single shared and
  reusable generator.

Fixes #5452
Fix problems with “database: true” on Teatro
Fixes #5457
label values for payment methods form

remove unused fullwidth class

It appears the button was loading as `<button class="fa fa-plus button"
name="button" type="{:class=>&quot;fullwidth&quot;}">Add</button>`
which isn’t very pretty.

bind numerous label tags to their inputs

add labels and move inputs onto single row

The quantity input was taking u pa whole row and so was the variant
input. A bit too long to look nice. Made it a lot more aesthetic and
add the label tags.

tab width with 2 spaces, not 4 spaced tab

use and instead of &amp;

the &amp; was not encoding, so I just switched to and.

add a page title for taxons

add taxons to products tab

update the adjustment button style

get more of the label tags matching

Fixes #5446
Fix regression from 9d12f0e
Update dependencies

* updated kaminari, cancancan and Ransack
* fix product admin page:
  delete 'deleted_at_null' from the searchg query hash as now ransack behaves
  property and won't include products with null delete_at value

Fixes #5455
fix spelling for destroy at various places
Fixes #5459
Add API endpoint /api/shipments/mine

With exchanges and split shipments we now have times when stores will be interested in querying a user's shipments directly rather than having to go through the user's orders.

Note: Trying to prevent a bunch of n+1 queries with a bunch of includes.

Fixes #5219
Make create endpoint require important params

bit of clean up on these tests, but no more after #create

Fixes #5458
Image configuration is no longer done.
Stop running next! before complete step to fix broken spec.
Moving admin index route to bottom of routes file to allow it to be overridden without breaking pagination on the orders page

Fixes #5461
Update jquery-ui-rails

Fixes #5462