Properly merge Tailwind CSS classes #985

You can pass additional classes to some Backpex components. As they often come with certain defaults, we need a way to properly merge overriding and default classes. If additional classes are passed to components, these classes should have a higher priority than default classes. Currently, you sometimes need to work with ! because default classes are of a higher priority (in terms of the place inside the final CSS file) than additional / overriding classes.