Add an option to disable account registation, and fix use of a JSContext from the wrong thread along the way.

core/core.js

@@ -79,6 +79,11 @@ const k_global_settings = {
 		default_value: false,
 		description: 'Enable discovery of, sharing of, and connecting to internet peer strangers, including announcing this instance.',
 	},
+	account_registration: {
+		type: 'boolean',
+		default_value: true,
+		description: 'Allow registration of new accounts.',
+	},
 };
 
 let gGlobalSettings = {