(bug 26857) Fatal error during installation enabling 'Vector' extension (or any exten...

(bug 26857) Fatal error during installation enabling 'Vector' extension (or any extension, really). The reason we're including extensions during setup is so they have a chance to register with LoadExtensionSchemaUpdates. Before including extensions, we now include DefaultSettings.php, so any operations on those variables won't result in the fatals described in the bug.

More importantly, this adds support for installing extension tables *at* install time. Previously, the best we could do was to add the require()s to LocalSettings. However, without a subsequent update, wikis will probably start having errors immediately after install (this is really bad!). So I've added an interface to the DatabaseUpdater passed to LoadExtensionSchemaUpdates. An extension can now call addNewExtension() which will allow the extension to be enabled on install and not just update. Example committed in CodeReview