Add Process Membership Checkout in a Modal or Popover Window

[dwanjuki]

Migrating snippet to library: https://gist.github.com/ideadude/d25fb89822f60143926071feace36812

Updated pmprocm_content() to check if $queried_object and post_content property is empty, resolves warnings:

strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated
Attempt to read property "post_content" on null

Also checking for existence of the Checkout Page block here. The original only checked for the pmpro_checkout shortcode.

Some WPCS changes and added recipe header.

[andrewlimaza]

Thanks for doing this one. 👍