Prevent default event on JavaScript-only links

These links point to "#" and don't do anything without JavaScript
activated, and they were causing the browser to scroll to the top of the
page.

app/assets/javascripts/markdown_editor.js

@@ -28,8 +28,10 @@
         editor.find("textarea").on("scroll", function() {
           editor.find(".markdown-preview").scrollTop($(this).scrollTop());
         });
-        editor.find(".fullscreen-toggle").on("click", function() {
+        editor.find(".fullscreen-toggle").on("click", function(e) {
           var span;
+          e.preventDefault();
+          e.stopPropagation();
           editor.toggleClass("fullscreen");
           $(".fullscreen-container").toggleClass("medium-8", "medium-12");
           span = $(this).find("span");