configitems: move rebase config into core The shelve extensions import and call rebase content without loading the extension. This is problematic as the config items rebase uses are not declared and the default value are not set, etc... The shelve extension should be using core utilities only and the necessary bit should be moved from rebase into core. In the meantime, I'm taking a small step to get config registration completed with minimal overhead. The rebase extension is shipped with core so registering its config option within core is not a big issue. This is the last step needed before we can install a warning that enforces all config to be registered.