Simplify the way we enqueue scripts and styles.

[pderksen]

It's too cryptic and hard to read and update in it's current state. For core and all add-ons lets enqueue on every page (like we have for other plugins) and document a way to dequeue in our code snippet library.

• Mimic what we've done in WP Simple Pay Lite & Pro.
• See https://github.com/moonstonemedia/WP-Simple-Pay-Library/blob/master/conditionally-load-scripts-styles.php
• Also do for admin pages, only enqueuing on our admin pages. Possibly other admin pages that needed (widgets, plugin listing, etc?).
• Do for all add-ons at same time.
• Also see Trello card.

[pderksen]

Gets a little more complex to get this right with enqueueing only on our admin pages. Let's tackle both public & admin pages at some point when this becomes an issue again.