css: avoid overflowing main-column
When we import HTML from foreign sources, we need to ensure that it behaves well with our design.
This fixes two details:
- Images are now responsive, so they won't grow beyond the parent element's width.
- Wide pre-formatted text now scrolls rather poke outside the parent element.
This makes the current content behave much better for me, and almost even makes this page kinda useful on mobile.