Consistently preserve anchors in the title specified in 'mainpage' message

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