Make dark theme a user level property, lazy load dark CSS

- Also now respects the colour-scheme media query
- Added meta tag to tell the browser we support dark theme, allowing dark UA stylesheet if the user sends said media query
- Means you only have to set it once per account rather than once per machine
- Dark themed embeds!

RIGS/templates/event_form.html

@@ -79,7 +79,7 @@
 
 {% block content %}
 {% include 'item_modal.html' %}
-<form class=" itemised_form" role="form" method="POST">
+<form class="itemised_form" role="form" method="POST">
     {% csrf_token %}
     <div class="row">
         <div class="col-12">