For Private page, add feature 'take note'

This makes this app much more meaningful. For this feature, a new database table is added. More complicated logics are also needed, for example, we use hash(user ID + timestamp + note contant) as the unique id of each note (it's necessary when we try to delete a note)

templates/page_403.html

@@ -0,0 +1,6 @@
+{% extends "layout.html" %}
+{% block page_title %}Forbidden(403){% endblock %}
+{% block body %}
+    {{ super() }}
+    This operation is forbidden.
+{% endblock %}