ng-message styling: add support for data- and x- prefixes

[Frank3K]

Commit a2bc3c6 introduced styling for ng-messages. Since the styling is on elements and (unprefixed) attributes (e.g. ng-message, [ng-message]) the styling does not work for prefixed attibutes. It would be nice if the styling is also applied on the prefixed variants of the ng-message attribute (e.g. [data-ng-message] and [x-ng-message]).

The change would (I think) only include changes to input-theme.scss and input.scss.

Related: #372