Fix close button position

The close button didn't stick to it's position after hitting
the first media-query towards mobile. The mobile close button
is not affected by this.
Caused by adjusting styles within this certain mq directly on
the .modal-close element rather on it's pseudo :after element
the correct values have now been applied.
The close button should now stick on it's position when the
module resizes.
Separate close button position from its theme styling

Because the current calculation of the close button
depends on the theme styling of the close button in
_modal-theme.scss it needs refactoring.
Based on the new calculation the styling of the close button
(for example padding) in _modal-theme.scss does not affect
the position anymore.
Fix modal modifier close button positions
Update changelog