Permalink Styling for YoURLs Controller

Duplicates https://github.com/norcross/yourls-link-creator/pull/3 for the benefit of the new repo

This pull implements a second UI mode that places the ShortURL input controller below a page's permalink input and styles it in a similar way:

 - Added a setting (and screenshots) to select the UI mode
 - Refactored (consolidated) input rendering code into two functions that respond based on UI mode
 - Added a function to render input for 'edit_form_after_title' region
 - Added logic to dynamically select add_action and wp_enqueue_style based on UI mode

The mode could use some further enhancement (especially CSS touch-up), but this lets you decide whether you're interested in integrating it into your repo before I make additional improvements.