Added Zotero socials (#1572)

Solving #1569 
I added Zotero socials because I needed them; opened a issue/pull
request in case I'm not the only one.

Second pull request (previous: #1570 ) because unwanted commits
retroactively crept in somehow. It was easier to force reset (which
closed the pull request automatically apparently).

_config.yml

@@ -104,6 +104,7 @@ instagram_id: # your instagram id
 facebook_id: # your facebook id
 youtube_id: # your youtube channel id (youtube.com/@<youtube_id>)
 discord_id: # your discord id (18-digit unique numerical identifier)
+zotero: # your zotero username
 
 contact_note: >
   You can even add a little note about which of these is the best way to reach you.

_includes/social.html

@@ -103,6 +103,9 @@
             {%- if site.discord_id -%}
             <a href="https://discord.com/users/{{ site.discord_id }}" title="Discord"><i class="fab fa-discord"></i></a>
             {% endif %}
+            {%- if site.zotero_username -%}
+            <a href="https://www.zotero.org/{{ site.zotero_username }}" title="Zotero"><i class="ai ai-zotero"></i></a>
+            {% endif %}
             {%- if site.rss_icon -%}
             <a href="{{ site.baseurl }}/feed.xml" title="RSS Feed"><i class="fas fa-rss-square"></i></a>
             {% endif %}