Consistently preserve anchors in the title specified in 'mainpage' message
When 'mainpage' was used in the 'sidebar' message, the anchor would be
preserved (code in Skin::addToSidebarPlain() uses Title::getLinkURL()).
When 'mainpage' was used on its own in Skin::makeMainPageUrl() (which
is used in SkinTemplate::buildNavUrls(), and many skins use that data
to generate the logo link), it would not be preserved (the code used
Title::getLocalURL()).
Bug: T176583
Change-Id: Icc4895c08089caaa06e37ce765819829b90de0ff