Autoload mu-plugins

Load standard plugins from within the mu-plugins folder.
```php
if ($screen->{'base'} != 'plugins' || $type != 'mustuse' || !current_user_can('activate_plugins')) {
  return $bool;
}
[...]
```

Thoughts?

Does the same thing except reduces indentation, and also makes it clearer that the method returns `$bool` if it doesn't do anything.