Add a Netlify badge to the footer

This will qualify us for Netlify's open source program.

source/_includes/footer.liquid

@@ -13,16 +13,25 @@
 
         {% renderFile 'source/_includes/footer_nav.md' %}
       </div>
-      <div class="sl-l-grid__column sl-l-large-grid__column--auto-size sl-l-large-grid--justify-right">
+      <div class="sl-l-grid__column sl-l-large-grid__column--auto-size sl-l-large-grid--justify-right footer-icons">
         <a
           class="twitter-follow-button"
           href="https://twitter.com/SassCSS"
           data-show-count="false"
           data-size="large">Follow @SassCSS</a>
+        <div class="sl-l-grid sl-l-small-grid--gutters">
+          <div class="sl-l-grid__column">
+        <a href="https://www.netlify.com"><img
+            alt="Deploys by Netlify"
+            src="https://www.netlify.com/v3/img/components/netlify-color-bg.svg"/></a>
+            </div>
+          <div class="sl-l-grid__column">
         <a href="https://www.macstadium.com/"><img
             alt="Powered by MacStadium"
             class="mac-stadium-icon"
             src="https://uploads-ssl.webflow.com/5ac3c046c82724970fc60918/5c019d917bba312af7553b49_MacStadium-developerlogo.png"></a>
+          </div>
+        </div>
       </div>
     </div>
   </div>

source/assets/sass/components/_footer-icons.scss

@@ -0,0 +1,15 @@
+.footer-icons {
+  max-width: 16rem;
+}
+
+.twitter-follow-button {
+  display: block;
+  margin-left: auto;
+  margin-right: auto;
+  text-align: center;
+  margin-bottom: var(--sl-gutter);
+}
+
+.mac-stadium-icon {
+  width: 7rem;
+}

source/assets/sass/sass.scss

@@ -28,7 +28,7 @@
 @use 'components/introduction';
 @use 'components/link-header';
 @use 'components/lists';
-@use 'components/mac-stadium-icon';
+@use 'components/footer-icons';
 @use 'components/pop-stripe';
 @use 'components/sass-syntax-switcher';
 @use 'components/tables';