Abstract references to social media accounts

lms/templates/index.html

@@ -47,13 +47,13 @@
             <div class="secondary-actions">
               <div class="social-sharing">
                 <div class="sharing-message">${_("Stay up to date with all {platform_name} has to offer!").format(platform_name=settings.PLATFORM_NAME)}</div>
-                <a href="https://twitter.com/edXOnline" class="share">
+                <a href="${settings.PLATFORM_TWITTER_URL}" class="share">
                   <img src="${static.url('images/social/twitter-sharing.png')}">
                 </a>
-                <a href="http://www.facebook.com/EdxOnline" class="share">
+                <a href="${settings.PLATFORM_FACEBOOK_ACCOUNT}" class="share">
                   <img src="${static.url('images/social/facebook-sharing.png')}">
                 </a>
-                <a href="https://plus.google.com/108235383044095082735/posts" class="share">
+                <a href="${settings.PLATFORM_GOOGLE_PLUS_URL}" class="share">
                   <img src="${static.url('images/social/google-plus-sharing.png')}">
                 </a>
               </div>