* Changed OutputPage's handling of subtitles to use an array and implode it with...

* Changed OutputPage's handling of subtitles to use an array and implode it with "<br />" so that callers don't have to bother whether there's already a subtitle when adding new ones
* Make OutputPage::setSubtitle(), OutputPage::addSubtitle() and OutputPage::appendSubtitle() accept a Message object in addition to a string
* Added new method OutputPage::addSubtitle() to replace appendSubtitle() and marked it as deprecated
* Clear the subtitles when showing an error page
* Always show the subtitle from Article::viewRedirect() when showing a page; left the parameter for use in EditPage
* Make Article::setOldSubtitle() add two subtitles and has to set "display:inline;" for the three possible divs to avoid too many spacing
* Removed the six different backlink subtitles and added one new message 'backlinksubtitle' to replace them and added OutputPage::addBacklinkSubtitle() to factorise common code
* Changed EditPage's view source to show "View source for <Page>" with the same backlink in subtitle for consistency with other back links and page titles