Make the layout adjust to narrow viewports better

Below about 600px (phones and small windowed browsers) the sidebar
starts breaking the layout. Moving it inline helps a lot; with
modern CSS (flex layout) we can also shuffle it after the main content.
Older browsers that don't do one of the 4 variants of flex layout
will just display it above the main content, which is annoying
but not too bad.

blueview also has a minimum width set so that it won't become
ridiculously small with small fonts, but that means it gets
scrollbars below about 850px; stop enforcing the minimum width
if necessary.

Finally, blueview's relatively generous margins look good in a
desktop browser, but are just too space-hungry on a phone.
Turn them off below 600px.
page.tmpl: tell mobile browsers we have a responsive layout, unless told not to

Mobile browsers typically assume that arbitrary web pages are
designed for a "desktop-sized" browser window (around 1000px)
and display that layout, zoomed out, in order to avoid breaking
naive designs that assume nobody will ever look at a website on
a phone or something. People who are actually doing "responsive
design" need to opt-in to mobile browsers rendering it at a
more normal size.
More page layout adjustments for mobile browsers and small windows
changelog and NEWS entry for responsive layout