Fix registration page

Also make it accessible by providing a mode argument to the install
function. Previously developers could just edit the url but we no longer
have the address bar as an app window, so now they must close the
default installer and run the following from the background page
console: `extension.install('standalone')`.

In the production build, this should result in an error since it is not
supported / the register page is not included there.

js/options.js

@@ -80,10 +80,8 @@
 
                 var accountManager = new bg.textsecure.AccountManager();
                 accountManager.registerSecondDevice(setProvisioningUrl, confirmNumber, incrementCounter).then(function() {
-                    extension.windows.getCurrent(function(appWindow) {
-                        bg.openInbox();
-                        extension.windows.remove(appWindow.id);
-                    });
+                    bg.openInbox();
+                    window.close();
                 });
             }
         });