Fix stock transfer translations

[Fixes #3030]
Delegate Spree.t to Rails TranslationHelper

This way we get all the nice features Rails t() helper offers, mainly
displaying reasonable strings for missing translations.

At this point core test suite should be all green. I've remove a couple
i18n specs which were translating to :en locale. I don't think that make
sense since :en is the default locale. Other translations specs pass now

[Fixed #3023]
Fix scss import.

[Fixes #2932]
Put variables back where it was.

[Fixes #2422]
add the template file for generating the .rspec file. this was causing
"Could not find "rspec" in any of your source paths. Your current source
paths are: " [Fixes #3031]
Add request spec for backend new order process [Fixes #3022]

Fix order details and shipping method request specs

[Fixes #3000]
[Fixes #3032] When OrderInventory#verify is given a specific shipment, only remove variant from that shipment
Test to display order number when order is shown
Less obfuscated check for order#completed? [Fixes #3034]
Reload shipment after removing variant item via api

Fix broken spec on api build. The item was being removed properly but
the controller still returned the old obj instead of the new one with
the item removed
Force select2-rails to 3.2.1

select2-rails 3.2.2 breaks all capybara helpers in Spree that depends on
the label tag for attribute. e.g.

    <%= label_tag :add_variant_id, Spree.t(:name_or_sku) %>

    Instead of:

    <label for="s2id_add_variant_id">some text</label>

    will generate:

    <label for="s2id_autogen1">some text</label>

see https://github.com/ivaynberg/select2/commit/519070b11da9347cf1542d26fc59a83b0c3fd934

wtf select2 ..

Most of backend request specs were failing due to this
Only reload shipment if it's persisted [Fixes #3037]

Fix remaining backend failing specs
Add index on spree_variants.sku [Fixes #3038]
display stock_transfer number on show page
missing translation for stock transfer show
Prevent adding items without stock via admin UI [Fixes #3041]

If stock item on hand is 0 and it doesn't allow backordering admin
shouldn't be able to add the product

[Fixes #2998]
Taxon edit form layout tidyup

Fixes #3042
Consolidate spree/order attributes for coupon_code
Add backorderable default config to StockLocation [Fixes #3045]

Stock items backorderable values will be set according to this config
every time either a variant or a stock location is created
Add propagate variants config to StockLocation [Fixes #3050]

Every time either a new stock location or new variant is created the
stock_location##propagate_all_variants value will be checked to decide
whether new stock items should be created for the new stock location or
variant