Add workaround for icons backwards compatibility

Now old pelicanconf.py should be rendered correctly with new Font Awesome:
- Some special cases are hardcoded. For now those are 'rss' and 'feed'
- If icon string starts with 'fa' it is assumed to represent full CSS class
- Otherwise a brand icon is assumed - sensible fallback for most users

alchemy/templates/include/header.html

@@ -1,3 +1,16 @@
+{%- macro fa(icon) -%}
+{%- set fa_special_icons = {
+  "feed": "fas fa-rss",
+  "rss": "fas fa-rss",
+} -%}
+{%- if icon in fa_special_icons -%}
+{{ fa_special_icons[icon] }}
+{%- elif icon.startswith("fa") -%}
+{{ icon }}
+{%- else -%}
+fab fa-{{icon}}
+{%- endif -%}
+{%- endmacro -%}
 <div class="row">
   {% if SITEIMAGE %}
     <div class="col-sm-4">
@@ -28,7 +41,7 @@
           {% if (LINKS or (DISPLAY_PAGES_ON_MENU and pages)) and loop.first %}
             <li class=" list-inline-item text-muted">|</li>
           {% endif %}
-          <li class="list-inline-item"><a class="{{ icon }}" href="{{ url(link) }}" target="_blank"></a></li>
+          <li class="list-inline-item"><a class="{{ fa(icon) }}" href="{{ url(link) }}" target="_blank"></a></li>
         {% endfor %}
       </ul>
     {% endif %}