Add NS_USER to MWNamespace::$alwaysCapitalizedNamespaces, as suggested by Happy-melon...

Add NS_USER to MWNamespace::$alwaysCapitalizedNamespaces, as suggested by Happy-melon on CR r57558. Untested, but should be beneficial, allowing easier linking to users on wikis with $wgCapitalLinks=false. But redirects from invalid names to valid names, e.g. [[User:tim]] -> [[User:Tim]] will become inaccessible. r57558 itself suffers similar problems, inaccessible pages should really be cleaned up with a maintenance script.