@media print hides the sidebar and footer

[Fil]

@media print hides the sidebar and footer, and reshapes the main section

closes #743

(to test, try to print [or print-to-pdf] in landscape mode, because the sidebar is already hidden in portrait mode)

before (chrome, landscape)	after

Browsers are smart enough to try and use light mode before printing. However if you set the theme to "dark" (only) they might use some sort of "inverted" mode. It works about OK in any case, but in some cases the DOT charts and the text copy might be a bit paler than expected.

Firefox looks a lot like Chrome above. Safari was particularly wrong: it wasn't seeing the correct dimensions and made a very thin column of text. This is also fixed now. (It's a bit painful to test since you have to restart the browser between tries.)

before (safari, portrait)	after

It would be nice to have several testers, there are probably situations and browsers and OS that need more fixes. But it's better than nothing.

[mbostock]

Looks good, but I would either move the media query to the import statement, or fold print.css entirely into layout.css (I think?) where we define the sidebar to keep sidebar-related styles together.