Forum topic navigation uses a different larrow/rarrow than activity navigation and breadcrumb

Test in Boost and Classic. Insert a forum in a course, where there is already a first activity preceeding and a third activity following. Create at least three topics in this forum. Go to the second one of that topic. Verify that the previous and next topic navigation items do look like the activity navigation items. Cross-check this for a RTL language, too. Note this patch also removes some unneeded left padding which could be previously seen on the "previous topic" navigation element and aligns forum topic navigation to activity navigation.