Fix detection of mail services for UI

Mail tab was not displayed if only exim was installed.

web/templates/admin/panel.html

@@ -68,7 +68,7 @@
 		<?php } ?>
 	<?php } ?>
 
-	<?php if ((isset($_SESSION['IMAP_SYSTEM'])) && (!empty($_SESSION['IMAP_SYSTEM']))) {?>			
+	<?php if ((isset($_SESSION['MAIL_SYSTEM'])) && (!empty($_SESSION['MAIL_SYSTEM']))) {?>
 		<?php if($panel[$user]['MAIL_DOMAINS'] != "0") { ?>
 					<div class="l-stat__col <?php if($TAB == 'MAIL' ) echo 'l-stat__col--active' ?>">
 						<a href="/list/mail/">

web/templates/user/panel.html

@@ -67,7 +67,7 @@
 		<?php } ?>
 	<?php } ?>
 
-	<?php if ((isset($_SESSION['IMAP_SYSTEM'])) && (!empty($_SESSION['IMAP_SYSTEM']))) {?>			
+	<?php if ((isset($_SESSION['MAIL_SYSTEM'])) && (!empty($_SESSION['MAIL_SYSTEM']))) {?>
 		<?php if($panel[$user]['MAIL_DOMAINS'] != "0") { ?>
 					<div class="l-stat__col <?php if($TAB == 'MAIL' ) echo 'l-stat__col--active' ?>">
 						<a href="/list/mail/">