Generate mobile bi-directional links

We have canonical links that point to the desktop site from mobile, but Google
would really like us to point the other way, too - to mobile from desktop.
They call these "bi-directional annotations".  More on that here:

    https://developers.google.com/webmasters/mobile-sites/mobile-seo/configurations/separate-urls?hl=en

r2/r2/lib/pages/pages.py

@@ -272,6 +272,10 @@ def __init__(self, space_compress=None, nav_menus=None, loginbox=True,
             if g.domain_prefix:
                 u.hostname = "%s.%s" % (g.domain_prefix, u.hostname)
             self.canonical_link = u.unparse()
+        # Generate a mobile link for Google.
+        u = UrlParser(request.fullpath)
+        u.hostname = 'i.%s' % g.domain
+        self.mobile_link = u.unparse()
 
         if self.show_infobar:
             if not infotext:

r2/r2/templates/base.html

@@ -35,6 +35,9 @@
     <meta name="description" content="${getattr(thing, 'short_description', None) or g.short_description}" />
     <meta name="referrer" content="${c.referrer_policy}">
     <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+    %if hasattr(thing, 'mobile_link'):
+        <link rel="alternate" media="only screen and (max-width: 640px)" href="${thing.mobile_link}" />
+    %endif
     ${self.viewport()}
     ${self.robots()}
     ${self.pagemeta()}