Use Spree.url() to build taxonomies jstree urls

URLs were hardcoded which caused errors when users mounted Spree in a
path other then "/"

[Fixes #3070] [Fixes #3071]
Sandbox generator now uses the 2-0-stable branch of spree_auth_devise
Installer should use stable branch of spree_auth_devise

[Fixes #3078]
Changed required RubyGems version to 1.8.23 so that Heroku can host it
Create a CHANGELOG file for each Spree component

It should help us to have a detailed idea of what changes between
versions. Both community and core developers could add an entry for
every relevant change. e.g. deprecated methods, relevant API
improvements, fixes.

Fixes #3062
Adding i18n values to English Locale
Refactor frontend address spec

This spec has failing on the CI for a while. The reason might be that it
used capybara `find` to check the visibility of elements and that method
does not wait until the element become either visible or not visible so
depending on the page load time the spec may get confusing results.

`has_selector?` is probably a better choice since we can specify
explicitly the `visible` option

[Fixes #3084]
Symbolize attachment styles keys on controller

[Fixes #3069] [Fixes #3080]
Insert changelog entries for recent commits
Remove order.update_totals from order.payment_required?

Instead we can call `order.update_totals` on the checkout_flow block
only since that's the only place on the code base where it seems to be
necessary. Plus calling `order.update_totals` generates a whole bunch of
work users may really not need or expect when calling
`order.payment_required?`

Fixes #3086
Fixes #3040
Improve formatting of 2-0-stable core changelog
Use change method to trigger use_billing_address

That should prevent the shipping address form being hidden when the
browser autocompletes the form.

[Fixes #3068]

Fixes #3085
Deprecate Order#shipment as split_shipments makes it less relevant
No need to pass option on spree.promo initializer

For some reason this was making the calculator.shipping_methods nil in
the spree app initializer. It prevented users from adding their own
calculators with code like:

  Rails.application.config.spree.calculators.shipping_methods << CustomUserClass

Fixes #3118
Remove nonsense js code on backend customer details

When picking up a user at the backend customer details order step that
code would empty all address fields, including the previously set
default country

This should fix the backend build.

Fixes #3129
Add nil object check for StockLocation#fill_status [Fixes #3131]
Fix product property delete url.

Fixes #3120 #3123
Remove leftovers allow_backorders references

Backorders can be configured now as per stock location or stock item

[Fixes #3049]
Fix remove taxon js code

[Fixes #3136][Fixes #3140]
Change InventoryUnit#backorderable_for_stock_item implementation

Users were getting ActiveRecord::ReadOnlyRecord when updating a stock
item with backordered units. Due to ActiveRecored `joins` return
readonly objects

[Fixes #3066]