Make padding around figures and tables more consistent

[aidhog]

Sometimes the padding around floating elements can be quite variable. In some cases the result looks quite cramped as per the space below the figure shown here:

and in this case above the figure:

Would be good to make the padding more generous and/or consistent. (In many cases it is quite okay.)

[Antoine-Zimmermann]

This is a bit tricky. I used a dirty CSS trick to make pictures fit nicely in almost all cases in a very simple and uniform way, but this trick also makes some picture look oddly placed. We could deal with each figure individually by moving them a bit or changing the padding, margin, etc. but this is a bit of work, and what is "nice" depends on the beholder.