feat!: move every layout to demo folder

- source/screens is now free for user files
- rename demo templates to their equivalent sources
- add demo maintenance template
- demo templates use html base element

source/screens/demo/pages/blank.njk

@@ -0,0 +1,20 @@
+<!-- blank.page -->
+{% set pageBase = "../" %}
+{% set pageId = "blank" %}
+{% set pageClass = "h_full_view" %}
+
+{% extends "demo/_default.njk" %}
+
+{% block title %}Blank{% endblock %}
+
+{% block head %}
+{{ super() }}
+{% endblock %}
+
+{% block body %}
+
+{% endblock %}
+
+{% block script %}
+{{ super() }}
+{% endblock %}