Multiple categories in the product_category shortcode

Add the ability to input multiple categories separated by a comma in the
[product_category] shortcode.
IT languages update
added copy for support button, closes #5865
Improve display of multiword product attributes

Instead of displaying a variation's custom product attributes like this:

`name-with-dashes: attribute-with-dashes`

Display it like this:

`Name With Dashes: Attribute With Dashes`
Add coupon description to REST API coupon response
Add shop_webhook custom post type

Part of #5564
Add webhooks REST API endpoint

Part of #5564
Introduce WC_Webhook class

Part of #5564
Hide webhook comments

Part of #5564
Add shop_webhook capabilities on install/upgrade

Part of #5564
Load webhooks on each page load

Part of #5564
Merge pull request #5938 from thenbrent/custom_attribute_label

Improve display of multiword product attributes
Send order as a whole to paypal with negative line items

Closes #5862
Merge pull request #5941 from maxrice/feature-issue-5564-rest-api-webhooks

Webhooks Implementation
Merge pull request #4632 from AdvancedStyle/master

Multiple categories in the product_category shortcode
More efficient #4632
Change shutdown hook priority

Fixes #5860
Include template functions after the theme is loaded, but before init.

Fixes conflicts with plugins (#5884) when they output content before
init.

2.2 has no fatal error as reported in #5884
Welcome page Closes #5920

@jameskoster can your review?
Pass is_ajax through from the ajax handler

Fixes #5910