Review CSS of some buttons

[stefanhaerter]

On places where we use anchor tags instead of real buttons, the span holding icon and text is slightly misplaced (better visible on higher zoom levels in browser) to the top, coming from asymmetric padding (3px top, 6px bottom). A possible fix is locally implemented in commit b9abc6a.

It needs to be checked if it is a profound idea to implement this fix in the general place for all of those buttons.