Make all the social links show icons

lms/templates/footer-edx-new.html

@@ -64,7 +64,7 @@
           <span class="copy">${_("Twitter")}</span>
         </a>
         <a href="${settings.PLATFORM_FACEBOOK_ACCOUNT}" title="Facebook">
-          <i class="icon fa fa-facebook-sign element-invisible"></i>
+          <i class="icon fa fa-facebook element-invisible"></i>
           <span class="copy">${_("Facebook")}</span>
         </a>
         <a href="${settings.PLATFORM_MEETUP_URL}" title="Meetup">
@@ -72,11 +72,11 @@
           <span class="copy">${_("Meetup")}</span>
         </a>
         <a href="${settings.PLATFORM_LINKEDIN_URL}" title="LinkedIn">
-          <i class="icon fa fa-linkedin-sign element-invisible"></i>
+          <i class="icon fa fa-linkedin element-invisible"></i>
           <span class="copy">${_("LinkedIn")}</span>
         </a>
         <a href="${settings.PLATFORM_GOOGLE_PLUS_URL}" title="Google+">
-          <i class="icon fa fa-google-plus-sign element-invisible"></i>
+          <i class="icon fa fa-google-plus element-invisible"></i>
           <span class="copy">${_("Google+")}</span>
         </a>
       </div>