Added rel=me to Mastodon (#308)

Mastodon uses this to verify a personal link. (https://docs.joinmastodon.org/usage/basics/#link-verification)

_includes/social.html

@@ -6,7 +6,7 @@
   {%- if site.instagram_username -%}<li><a href="https://www.instagram.com/{{ site.instagram_username| cgi_escape | escape }}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#instagram' | relative_url }}"></use></svg> <span class="username">{{ site.instagram_username| escape }}</span></a></li>{%- endif -%}
   {%- if site.linkedin_username -%}<li><a href="https://www.linkedin.com/in/{{ site.linkedin_username| cgi_escape | escape }}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#linkedin' | relative_url }}"></use></svg> <span class="username">{{ site.linkedin_username| escape }}</span></a></li>{%- endif -%}
   {%- if site.pinterest_username -%}<li><a href="https://www.pinterest.com/{{ site.pinterest_username| cgi_escape | escape }}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#pinterest' | relative_url }}"></use></svg> <span class="username">{{ site.pinterest_username| escape }}</span></a></li>{%- endif -%}
-  {%- for mst in site.mastodon -%}{%- if mst.username and mst.instance -%}<li><a href="https://{{ mst.instance| cgi_escape | escape}}/@{{mst.username}}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#mastodon' | relative_url }}"></use></svg> <span class="username">{{ mst.username|escape }}</span></a></li>{%- endif -%}{%- endfor -%}
+  {%- for mst in site.mastodon -%}{%- if mst.username and mst.instance -%}<li><a rel="me" href="https://{{ mst.instance| cgi_escape | escape}}/@{{mst.username}}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#mastodon' | relative_url }}"></use></svg> <span class="username">{{ mst.username|escape }}</span></a></li>{%- endif -%}{%- endfor -%}
   {%- if site.twitter_username -%}<li><a href="https://twitter.com/{{ site.twitter_username| cgi_escape | escape }}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#twitter' | relative_url }}"></use></svg> <span class="username">{{ site.twitter_username| escape }}</span></a></li>{%- endif -%}
   {%- if site.youtube_username -%}<li><a href="https://www.youtube.com/{{ site.youtube_username| cgi_escape | escape }}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#youtube' | relative_url }}"></use></svg> <span class="username">{{ site.youtube_username| escape }}</span></a></li>{%- endif -%}
   {%- if site.youtube_channel -%}<li><a href="https://www.youtube.com/channel/{{ site.youtube_channel| cgi_escape | escape }}"><svg class="svg-icon grey"><use xlink:href="{{ '/assets/minima-social-icons.svg#youtube' | relative_url }}"></use></svg><span class="username">{%- if site.youtube_channel_name -%}{{ site.youtube_channel_name | escape }}{%- else -%}YouTube{%- endif -%}</span></a></li>{%- endif -%}