Restore "onion-location" meta tag

See:
- https://forum.qubes-os.org/t/add-tor-onion-location-to-qubes-os-org/5919
- beccc81#r55472194
- #153
- #135

Author: andrewdavidwong

_includes/head.html

@@ -17,6 +17,7 @@
   <meta charset="utf-8">
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
   <meta name="viewport" content="width=device-width, initial-scale=1">
+  <meta http-equiv="onion-location" content="http://qubesosfasa4zl44o4tws22di6kepyzfeqv3tg4e3ztknltfxqrymdad.onion{{ page.url | replace:'index.html','' }}">
   <!-- Page title -->
   <title>
     {% if page.title and page.title != site.title and page.layout != 'home' %}