Remove the flicker when switching from pick email to add new email.

close #410

browserid/static/dialog/css/popup.css

@@ -512,6 +512,10 @@ html[xmlns] .cf {
     overflow-y: auto;
 }
 
+.pickemail .form_section {
+    height: 176px;
+}
+
 .add {
     font-size: 80%;
 }

browserid/static/dialog/views/pickemail.ejs

@@ -13,7 +13,7 @@
           <div class="vertical">
               <strong>Sign in using</strong>
 
-              <div id="selectEmail">
+              <div id="selectEmail" class="form_section">
                   <ul class="inputs">
                       <% _.each(identities, function(email_obj, email_address) { %>
                           <li>
@@ -34,7 +34,7 @@
                   </div>
               </div>
 
-              <div id="addEmail" class="cf">
+              <div id="addEmail" class="cf form_section">
 
                   <ul class="inputs">
                       <li>