Execute `woocommerce_order_action_{$action}` only once

Prevents a “Maximum function nesting level reached” fatal error in
certain cases.

Since this ends up running on `’save_post’`, updating something like
the order status while hooked onto to
`woocommerce_order_action_{$action}` causes a fun recursion :)
add display_prices in variation data

The commit introduces 3 changes:
1. It adds 2 extra fields to the available_variations array,
display_price and display_regular_price, to be used by extensions in
cases like
https://github.com/woothemes/woocommerce-product-addons/issues/19
2. Introduces a get_display_price() function in WC_Product to retrieve
product prices incl or excl tax relative to the
woocommerce_tax_display_shop setting.
3. Fixes a minor issue where passing a 0 price to
get_price_including_tax() or get_price_excluding_tax() results in a
wrong output.
Fixed the wc_update_new_customer_past_orders() function, closes #6903

get_post_status() returns post status (string) or false (bool) on failure.
Fixed the coupons tests and improved the tax tests
clickable item name in orders table
add order_item_visible_filter

In line with the existing 'cart_item_visible' filter.
Merge branch 'breadcrumbs-class'

Adds endpoint page titles for #5522
Fixes link back to endpoint pages and closes #6638
Avoids need for breadcrumb fix which closes #6834
Removes the need and closes #6834
Fixes #6840
Adjust wc_timezone_string to use is_dst and return the timezone when not false. Only fallback to UTC when needed.

Fixes #6897
Merge pull request #6908 from franticpsyx/order_item_visible_filter

[2.3] Add order_item_visible_filter
Merge pull request #6905 from SiR-DanieL/patch-2

Clickable item name in orders table
Merge pull request #6875 from tamarazuk/limit-order-action

Execute `woocommerce_order_action_{$action}` only once
Fix timezone_name_from_abbr unit test
Correctly show terms when attribute has outfit chars Closes #6872
Fix purchase note output

Fixes #6835
Delete cached sessions when clearing expired sessions
Fixes #6907
only delete sessions from object cache if using persistent object cache
ajaxified coupon removal on checkout page for better UX closes #6881
Merge pull request #6911 from splashingpixels/checkout-coupons

ajaxified coupon removal on checkout page for better UX closes #6881
Merge pull request #6909 from webaware/cleanup-sessions-cached

Delete cached sessions when clearing expired sessions
Retrive reviewers avatar using the email

Using the email will fix the wrong avatar issue occurring while creating manually the review from the admin panel.