Refactor #52 with easier, functional support for DFP Async.

Some key ideas:
* DFP Async default configuration lives in doubleclick-for-publishers-async.php. The goal is for it to be comprehensive enough that all you need to do is do_action() for your ad tags. The last things to do are standardize the ad code configuration variables and default ad units.
* 'enable_ui_mapping' is a new flag you can add to your ad tags. Functionally, it means a given ad code mapped through the admin UI to an ad tag will only be registered with that tag.
* get_matching_ad_code() is a new class-level method to get the currently matching ad code for a tag. It's an abstract
ion of the conditional logic in action_acm_tag() with nothing additional added.
* Added support for `<select>` type URL vars in Quick Edit. @aliso had already added this for the "New Ad Code" form.