Add option to add custom CSS class to <div id="klaro">

Using Simple Klaro in a multi-site setup it's often necessary to style the pop-up slightly different per instance. This of course can be achieved in multiple ways with custom code. It would be nice though to have the possibility to add a custom CSS class to the <div id="klaro"> DIV directly. For example <div id="klaro" class="my-custom-class"> for example. This makes styling per multi-site instance much easier.

1. Extend config to add new field "Custom CSS class", empty by default, description "Add a custom CSS class to the modal DIV".
2. Validate new field for being a valid CSS class.
3. Add this CSS class to the <div id="klaro"> DIV