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/templates/demo/_default.njk

@@ -0,0 +1,18 @@
+<!-- demo.default.template -->
+{% extends "hippie/_default.njk" %}
+
+{% block meta %}
+  {% include "demo/partials/_meta.njk" %}
+  <base href="/">
+{% endblock %}
+
+{% block links %}
+  {{ super() }}
+  {# <link rel="stylesheet" type="text/css" media="all" href="css/demo.css"/> #}
+  {# <link rel="stylesheet" type="text/css" media="all" href="{{ pageBase | subdir(2) }}css/demo.css"/> #}
+{% endblock %}
+
+{% block script %}
+  {{ super() }}
+  <script src="https://code.jquery.com/jquery-3.3.1.min.js" integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8=" crossorigin="anonymous"></script>
+{% endblock %}