* Made the web upgrade process more friendly. Instead of saying "access denied, go...

* Made the web upgrade process more friendly. Instead of saying "access denied, go away" when the user has a normal LocalSettings.php file, generate a random $wgUpgradeKey and instruct the user to insert it into their LocalSettings.php. The subsequent file modification then authenticates the session and allows the upgrade.
* When an upgrade key is entered, or a supplied upgrade key is edited into LocalSettings.php by the upgrader, fetch database settings from LocalSettings.php and AdminSettings.php for use during the upgrade. This allows the DBConnect page to be skipped, making web upgrade almost as easy to use as CLI upgrade.
* Made LocalSettingsGenerator add $wgUpgradeKey in non-commented form, for easier subsequent upgrades.
* Converted the $wgUpgradeKey check to a normal in-sequence page, called ExistingWiki. This allows the removal of related special cases from WebInstaller. The code for WebInstaller_ExistingWiki is loosely based on WebInstaller_Locked.
* Added Status::replaceMessage(), to support informative DB connection error messages from the ExistingWiki page.
* In WebInstaller::getInfoBox(), call parse() with $lineStart=true, so that line-start syntax like bullet points can work.
* Reduced the length of the generated $wgUpgradeKey from 64 to 16. This is ample for what it does, and makes it fit on the screen and not overlap with the right sidebar when when displayed by WebInstaller_ExistingWiki.
* Added $wgUpgradeKey to DefaultSettings.php.