Use change event instead of blur.

This solves a bug whereby switching to the register form doesn't happen because
validation gets triggered which somehow prevents it.

src/converse-controlbox.js

@@ -462,7 +462,7 @@
                 className: 'controlbox-pane fade-in',
                 events: {
                     'submit form#converse-login': 'authenticate',
-                    'blur   input': 'validate'
+                    'change input': 'validate'
                 },
 
                 initialize (cfg) {

src/converse-register.js

@@ -403,9 +403,6 @@
                     return form;
                 },
 
-                showRegistrationForm () {
-                },
-
                 showSpinner () {
                     const form = this.el.querySelector('form');
                     form.innerHTML = tpl_spinner();