disable username registration field when needed

when the register your username checkbox is not ticked, disable the text
entry field.

Also clear some redundant labels and fix migration string asking for
password.

Change-Id: I30de073eb90da47ced1b54cbfebe2f7a10e8ebb5
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>
diff --git a/wizarddialog.h b/wizarddialog.h
index e2984e1..a2baa2d 100644
--- a/wizarddialog.h
+++ b/wizarddialog.h
@@ -65,6 +65,7 @@
     void handle_nameRegistrationEnded(NameDirectory::RegisterNameStatus status, const QString& name);
     void timeoutNameLookupTimer();
     void on_photoTaken(QString fileName);
+    void on_signUpCheckbox_toggled(bool checked);
 
 private:
     Ui::WizardDialog* ui;