WooCommerce One Page Checkout 2.8.1

* New: Introduce helper to find if it's a frontend request
* New: Bumps WC tested up to version.
* Tweak: Updates notice copy to reflect minimum WC version
* Tweak: Update minimum required WC version
* Tweak: Removes WC 2.5 pre code
* Tweak: Remove duplicate jQuery selectors
* Fix: Hides shipping fields for virtual products
* Fix: Avoid duplicated notices when variables product is not selected
* Fix: Prevent errors while generating page via Gutenberg editor
* Fix: Removes use of deprecated jQuery function size()

* New: Add documentation to the README file for `category_ids` shortcode property. PR #338
* New: Allow disabling the auto-scroll behavior when cart items are added/removed/changed. PR #339
* New: Allow passing custom data to Ajax calls on easy pricing table templates. PR #326
* New: Include category IDs in the shortcode generator (requires WC 3.2+). PR #342
* Fix: Compatibility with Name Your Price hook changes. PR #324
* Fix: [WC 3.2] Ensure `no-index` meta is not displayed on OPC pages. PR #336
* Fix: Do not use `__FILE__` PHP constant for nonce action names. PR #325
* Fix: Only display shipping fields on single product pages when a child product requires shipping. PR #335
* Fix: Include simple children of grouped products on OPC pages. PR #330

* Fix: [WC3.0] Get a variation product parent product ID is a WC version compatible way. PR#290
* Fix: Display the list of product names added to the cart when using add-to-cart url parameters. PR#302
* Fix: Update the `woocommerce_cart_item_quantity` filters to include the cart item argument. PR#311
* Fix: Update the `woocommerce_cart_item_quantity` filters to include the cart item argument. PR#311
* Fix: Fix display issues when displaying products with long names in the One Page Checkout edit page modal. PR#315
* Fix: Ensure Ajax request URLs use relative format to avoid issues when the site is `https` but the current page is `http`. PR#310
* Fix: Search for products by visibility in a WC version compatible way. Fixes an issue were OPC pages using category_ids shortcode args were not displaying any products.