Added opt in/out page for Mercury

This is a temporary Special page to enable opt-in/out for the Mercury Internal test
No need for this constructor to have the same signature as the parent. Also, since the parent defines defaults which it looks like you want, you don't have to add them again. I'd clean this up like this:
```php
pubic function __construct(){
    parent::__construct( self::PAGE_NAME );
}
```
and remove ```const PAGE_RESTRICTION = '';``` on line 10.