Fix incorrect button rounding (#4862)

.btn-group uses :first-child and :last-child to determine which
buttons should have rounded corners and which ones should have
square corners. The "Quiet" button had a square left corner,
because the first child was a CSRF hidden field tag.

Keeping only .btn elements inside the .btn-group means that the
1 or 2 buttons are rounded as expected. When there's no "Quiet"
button, the "Stop" button has 4 round corners. When both buttons
are present, the corners between the buttons are square.

Fixes #4858

web/views/busy.erb

@@ -78,9 +78,10 @@
         <td><%= process['busy'] %></td>
         <td>
           <form method="POST">
+            <%= csrf_tag %>
+            <input type="hidden" name="identity" value="<%= process['identity'] %>"/>
+
             <div class="btn-group pull-right flip">
-              <%= csrf_tag %>
-              <input type="hidden" name="identity" value="<%= process['identity'] %>"/>
               <% unless process.stopping? %><button class="btn btn-warn" type="submit" name="quiet" value="1"><%= t('Quiet') %></button><% end %>
               <button class="btn btn-danger" type="submit" name="stop" value="1"><%= t('Stop') %></button>
             </div>