Change imagemagick optional to depend on one variable (#745)

_config.yml

@@ -240,10 +240,8 @@ filtered_bibtex_keywords: [abbr, abstract, arxiv, bibtex_show, html, pdf, select
 # Responsive WebP Images
 # -----------------------------------------------------------------------------
 
-enable_responsive_images: true # enables responsive images for your site (recomended, see https://github.com/alshedivat/al-folio/issues/537)
-
 imagemagick:
-  enabled: true # make it 'false' if not using responsive images
+  enabled: false # enables responsive images for your site (recomended, see https://github.com/alshedivat/al-folio/issues/537)
   widths:
     - 480
     - 800

_includes/figure.html

@@ -3,7 +3,7 @@
 <figure>
 
   <picture>
-    {% if site.enable_responsive_images %}
+    {% if site.imagemagick.enabled %}
     {% for i in site.imagemagick.widths -%}
       <source media="(max-width: {{ i }}px)" srcset="{{ path | relative_url }}-{{ i }}.webp" />
     {% endfor -%}