layout: harmonize margins around main
We're currently YOLOing the margins and padding around main pretty badly, leading to slight inconsistencies between the different page-types. This makes it consistent; we now have a margin of 3 (which is bootstrap for 1rem) around all main-tags. To render more or less the same, we need to make some page-specific adjustments, like reducing a jumbotron-margin on the home-page. The only functional change should be the margin at the bottom of the 404-page, which is not smaller, like on other pages. It seems better to be consistent here.