Socials icons: Check if rss_icon: false (alshedivat#3282)

Partially addresses issue alshedivat#3280.

Currently, the RSS feed icon in the socials section will show whenever
`rss_icon` in `data/_socials.yml` has a value --- **even if that value
is `false`**.

Unlike the other social settings, which are empty by default, for
`rss_icon` the default setting is `true`.

So it makes sense that a user might expect `rss_icon: false` to disable
said icon.

This PR just adds a check if the value is `false` and hides the icon if
so.

Author: scottleechua

_includes/social.liquid

@@ -59,7 +59,9 @@
     {% when 'research_gate_profile' %}
       <a href="https://www.researchgate.net/profile/{{ social[1] }}/" title="ResearchGate"><i class="ai ai-researchgate"></i></a>
     {% when 'rss_icon' %}
-      <a href="{{ site.baseurl }}/feed.xml" title="RSS Feed"><i class="fa-solid fa-square-rss"></i></a>
+      {% if social[1] == true %}
+        <a href="{{ site.baseurl }}/feed.xml" title="RSS Feed"><i class="fa-solid fa-square-rss"></i></a>
+      {% endif %}
     {% when 'scholar_userid' %}
       <a href="https://scholar.google.com/citations?user={{ social[1] }}" title="Google Scholar"><i class="ai ai-google-scholar"></i></a>
     {% when 'scopus_id' %}