Find a solution for top padding with sticky header

[davidknezic]

When someone uses the sticky header header-fixed-top, the header becomes positioned absolute.

This makes it overlap content. This is why the content has to be padded to the top, to guarantee space for the header. We should find a solution where people aren't forced to provide inline styles or extend their css to add that padding.

We could use utilities for that, like the already existing push-{breakpoint}-left, but since the header height is quite unpredictable that makes it kinda hard.