Refactor: Modularized footer.njk and header-footer.css

- Split header-footer.css into:
    - header.css
    - footer.css
    - modals.css
- Split footer.njk into:
    - footer.njk
    - modals.njk
- Updated front matter of:
    - index.njk
    - about.njk
    - credits.njk
    - gallery.njk
    - community.njk

src/_includes/base.njk

@@ -38,6 +38,8 @@
 
         {% include "footer.njk" %}
 
+        {% include "modals.njk" %}
+
         {% if pageScripts %}
             {% for script in pageScripts %}
                 <script src="{{ script }}"></script>