Modal buttons

References DoSomething/dosomething#1930, DoSomething/dosomething#1985.
For review: @DoSomething/front-end.

### Modal Close Buttons
Close buttons can now be added programmatically two ways:
 - Using a `data-modal-close` attribute on the modal template.
 - Specifying the `closeButton` optional argument when using `NEUE.Modal.open()` (takes priority over data attribute).
Valid options are "true", "yes", or "1" (show close button, and allow user to click on overlay to close modal); or "skip", which prevents user from closing modal by clicking the overlay and fires a form action on clicking "skip".

### Misc. Changes
 - Brought modal style overrides from the DS app into Neue.
 - Cleaned up modal SCSS.