Make sure login form can't be submitted too early

Tyhe sign in form should not be submitted before the submit event is
ready to be intercepted. This can occur if the user clicks too fast on
the sign in button.

cms/templates/login.html

@@ -17,7 +17,7 @@ from django.utils.translation import ugettext as _
     </header>
 
     <article class="content-primary" role="main">
-      <form id="login_form" method="post" action="login_post">
+      <form id="login_form" method="post" action="login_post" onsubmit="return false;">
 
         <fieldset>
           <legend class="sr">${_("Required Information to Sign In to {studio_name}").format(studio_name=settings.STUDIO_NAME)}</legend>