Merge #706: Add Onion-Location meta tag

d556c10 Add Onion-Location HTTP header (Riccardo Masutti)

Pull request description:

  Starting with the release of [Tor Browser 9.5](https://blog.torproject.org/new-release-tor-browser-95), websites can have their alternate .onion addresses advertised to Tor desktop users who have the 'Onion Location' option enabled.

  Sites that add the .onion address advertisement HTTP header can prompt their visitors to switch to a version delivered using the Onion service for improved security.

  Final result:
  ![ezgif-2-354636e604ed](https://user-images.githubusercontent.com/46527252/83661669-bced5700-a5c6-11ea-9416-789ff30e52ef.gif)

ACKs for top commit:
  harding:
    ACK d556c10 Reviewed code (all 1 line), checked site diff, and tested with Tor Browser via a temporary deployment.

Tree-SHA512: d6349ba2a63081a7cf2abb7cbe2be64a3587633132aa356a79b1320b0ab5977a72c9a111cd43bacfd728f86fdbd7fb472f00a7c86d7d48a4a5f7c4138d144ea0

Author: harding

_includes/head.html

@@ -11,6 +11,7 @@
 <title>{{ site.title | xml_escape }}{{ custom | xml_escape }}{% if title %} :: {{ title | xml_escape }}{% endif %}</title>
 {% if page.excerpt %}<meta name="description" content="{{ title | strip_html | xml_escape }}">{% endif %}
 <meta name="keywords" content="{{ page.tags | join: ', ' }}">
+<meta http-equiv="onion-location" content="http://6hasakffvppilxgehrswmffqurlcjjjhd76jgvaqmsg6ul25s7t3rzyd.onion{{ page.url }}" />
 {% if page.author %}
   {% assign author = site.data.authors[page.author] %}{% else %}{% assign author = site.owner %}
 {% endif %}