Redirection plugin

Chamilo plugin for the redirection of specific users after they login and redirect from the index.php to the selected URL.

Requires the addition of the following in configuration.php:
```
$_configuration['plugin_redirection_enabled'] = true;
```

This setting is defined in configuration.php rather than in settings_current to reduce the load, as it is used on every login.

Add the plugin in the "menu administration" region.

@TODO Check the load difference for just checking it in settings_current rather than building the plugin object