Add Facebook link to the header

Fixes #776

assets/css/screen.css

@@ -274,6 +274,11 @@ production stylesheet in assets/built/screen.css
 .gh-social a {
     opacity: 0.8
 }
+
+.gh-social a + a {
+    margin-left: 0.8rem;
+}
+
 .gh-social a:hover {
     opacity: 1;
 }
@@ -284,6 +289,11 @@ production stylesheet in assets/built/screen.css
     fill: #fff;
 }
 
+.gh-social-facebook svg {
+    height: 20px;
+    width: 20px;
+}
+
 a.gh-head-button {
     display: block;
     padding: 8px 15px;

default.hbs

@@ -43,6 +43,9 @@
             </div>
             <div class="gh-head-actions">
                 <div class="gh-social">
+                    {{#if @site.facebook}}
+                        <a class="gh-social-facebook" href="{{facebook_url @site.facebook}}" title="Facebook" target="_blank" rel="noopener">{{> "icons/facebook"}}</a>
+                    {{/if}}
                     {{#if @site.twitter}}
                         <a class="gh-social-twitter" href="{{twitter_url @site.twitter}}" title="Twitter" target="_blank" rel="noopener">{{> "icons/twitter"}}</a>
                     {{/if}}