TODO comments on the registration flow

Add some notes to Signup and the Registration component that it would be nice
to factor out some common code someday.

src/components/structures/login/Registration.js

@@ -28,6 +28,10 @@ var CaptchaForm = require("../../views/login/CaptchaForm");
 
 var MIN_PASSWORD_LENGTH = 6;
 
+/**
+ * TODO: It would be nice to make use of the InteractiveAuthEntryComponents
+ * here, rather than inventing our own.
+ */
 module.exports = React.createClass({
     displayName: 'Registration',