Multiple categories in the product_category shortcode

Add the ability to input multiple categories separated by a comma in the
[product_category] shortcode.
Added description parameter to the woocommerce_form_field function

With this it is possible to add a short description below every
checkout billing and shipping input field.
Ability to add custom quantity using shortcode
Update class-wc-shortcodes.php
Update add-to-cart.php
This branch is now 2.1.9-bleeding
fix case-insensitive matching for coupon posts with uppercase chars
Make the welcome page RTL compatible. Fixes #5405
Sanitize, but decode. UTF-8 Friendly. Fixes #5401

Originally changed 34db5d68b6c4d4376f16f559c01c41c3f5b6a60b
Tidyup brackets
Add tool for disabling shipping rate cache for debug
Stop sending line items to Mijireh Fixes #5365

Like PayPal, Mijireh struggles with out prices including tax due to
rounding errors. Since the validation cannot be disabled, its better to
just send the order as 1 item. This will prevent rounding errors and
payment failures.

Prices excluding tax are unaffected.
Fix fee/coupon lines typo in REST API order response
COD: no need to hide in admin

Also fixes a fatal error when WC()->payment_gateways()->get_available_payment_gateways() is called in the admin
Update abstract-wc-shipping-method.php

The order is backward so if you use the default 'is_available' function for your extensions, the 'excluding' option will always deny every country because this line returns an empty array.
Closes #5424
Add $package to is_available shipping method hooks
Revise how variation attributes are deleted/updated

Prevents issues with WPE caching when you delete and then update right
after.

Conflicts:

	includes/admin/post-types/meta-boxes/class-wc-meta-box-product-data.php
Trim commas and empty lines off address formats Fixes #5411
param comment Fixes #5408