Use 'profileiconbyid' icon for the user's picture in their sideblock.

This image can be sent with a long expires header, so given that it is on every page this makes sense.

Even if the user changes their profile icon, the image shown to them will still be correct, because it will be at a different URL now.

htdocs/lib/mahara.php

@@ -1787,6 +1787,7 @@ function profile_sideblock() {
     require_once('group.php');
     $data = array(
         'id' => $USER->get('id'),
+        'profileicon' => $USER->get('profileicon'),
     );
     $saveduser = $USER->get('parentuser');
     if (!empty($saveduser) && $saveduser->name) {

htdocs/theme/default/templates/sideblocks/profile.tpl

     <h3><a href="{$WWWROOT}user/view.php?id={$data.id}">{$data.id|display_name|escape}</a></h3>
-    <div class="fr"><a href="{$WWWROOT}user/view.php?id={$data.id}"><div class="center" style="width: 50px; height: 50px;"><img src="{$WWWROOT}thumb.php?type=profileicon&amp;maxwidth=50&amp;maxheight=50&amp;id={$data.id}" alt=""></div></a></div>
+    <div class="fr"><a href="{$WWWROOT}user/view.php?id={$data.id}"><div class="center" style="width: 50px; height: 50px;"><img src="{$WWWROOT}thumb.php?type=profileiconbyid&amp;maxwidth=50&amp;maxheight=50&amp;id={$data.profileicon}" alt=""></div></a></div>
     <ul class="sidebar-ul-title">
         {if $data.unreadnotifications}
         <li>