Minor GTK and HTML CSS tweaks.

ui/conversation-message.ui

@@ -501,6 +501,9 @@
                 <property name="position">0</property>
               </packing>
             </child>
+            <style>
+              <class name="geary-message-body"/>
+            </style>
           </object>
         </child>
       </object>

ui/conversation-web-view.css

@@ -6,6 +6,10 @@
  * General HTML style.
  */
 
+* {
+  transition: height 0.25s;
+}
+
 html {
   /* Trigger CSS 2.1 § 10.6.7 to get a shrink-wrapped height. */
   position: absolute !important;