make short asks less ugly

Jake’s test exposed this issue

frontend/templates/download.html

@@ -23,7 +23,7 @@ $j(document).ready(function() {
 <div id="lightbox_content">
 
 {% if show_beg %}
-<div class="border" style="padding: 10px;">
+<div class="border" style="padding: 10px; height: 18em">
     <div id="begblock" class="rh_beg">
         <div>Please help us thank the creators for making this book free. The amount is up to you.</div>
         <form class="begform" method="POST" action="{% url thank work.id %}#">