General Handler for Whitelisted Options' Submissions

[WraithKenny]

As stated over on #18285 WordPress should move away from posting to options.php. In order to do that, the Settings API needs a general purpose function that can be safely called on all Settings Pages that can handle posts to itself (generally referred to as 'take_action' in various places) and can handle what options.php currently does.

[WraithKenny]

For what would be involved, see ryan's work over at #18285 or my own take at ​https://github.com/unFocus/Scripts-n-Styles/blob/master/includes/class-sns-form.php#L103