feature #392, authentication keys, album notification

* On album notification (for a group), sends one distinct email for each user
  with a new authentication key.

* When someone clicks the link with auth=<key> in URL, if the user is not
  already connected, Piwigo will automatically connect the user.

themes/default/template/mail/text/html/cat_group_info.tpl

@@ -1,6 +1,6 @@
 <div id="cat_group_info">
 <h2>{'Informations'|@translate}</h2>
-<p>{$IMG_URL}</p>
+<p><a href="{$IMG.link}" class="thumblnk"><img src="{$IMG.src}"></a></p>
 <p>{'Hello,'|@translate}</p>
 <p>{'Discover album:'|@translate} <a href="{$LINK}">{$CAT_NAME}</a></p>
 <p>{$CPL_CONTENT}</p>