Have Wikicode store a title instead of trying to parse it

Because of T324431, HTML is somtimes ending up in the <title> element, so it isn't usable as an actual title. In most cases, we know the title since we requested it from the server, so just store that title since we at least know it's the correct title.

Most importantly this makes transform_to_wikitext() work for pages with customized titles.

Bug: T326490