prevent form submission with [Enter] key (Issue #83)

www/maposmatic/templates/maposmatic/new.html

@@ -150,7 +150,7 @@ $('#error-modal').modal('show')
   </div>
 </div>
 
-<form id="mainfrm" method="post" enctype="multipart/form-data" action="{% url "new" %}#submitmapform">
+<form id="mainfrm" method="post" enctype="multipart/form-data" onkeydown="return event.key != 'Enter';" action="{% url "new" %}#submitmapform">
   {{ form.mode }}
   <div class="row" style="margin-top: 30px;" >
     <div class="col-lg-12" >