Allow to run setup on the demo server

Fixes #13083

Signed-off-by: Michal Čihař <michal@cihar.com>

doc/config.rst

@@ -3214,6 +3214,12 @@ Developer
     Enable to let server present itself as demo server.
     This is used for `phpMyAdmin demo server <https://www.phpmyadmin.net/try/>`_.
 
+    It currently changes following behavior:
+
+    * There is welcome message on the main page.
+    * There is footer information about demo server and used git revision.
+    * The setup script is enabled even with existing configuration.
+
 .. _config-examples:
 
 Examples

setup/index.php

@@ -12,7 +12,7 @@
  */
 require './lib/common.inc.php';
 
-if (file_exists(CONFIG_FILE)) {
+if (file_exists(CONFIG_FILE) && ! $cfg['DBG']['demo']) {
     PMA_fatalError(__('Configuration already exists, setup is disabled!'));
 }