Fine-tuning small screen layout.

With #10 (closed) soon out of the way, there are still smaller things:

• The Kwant workflow image looks tiny when scaled; it can probably be reworked to use the grid system.
• In /community, the mailing list sign up forms don't fit into <li>
• Many images in / are too large at the upper border of small screen sizes (they take full width).
• Some text in / is too wide at intermediate screen size ~900px (reaches the border). EDIT: Fixed via !10 (merged).

EDIT: Added the last item.