page.tmpl: tell mobile browsers we have a responsive layout, unless told not to

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.