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!

users/templates/partials/navbar_user.html

@@ -3,22 +3,17 @@
     <a class="nav-link dropdown-toggle" href="#" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
         Hi {{ user.first_name }}
     </a>
-    <ul class="dropdown-menu p-3 clearfix" id="userdropdown">
+    <ul class="dropdown-menu clearfix" id="userdropdown">
         <li class="media">
             <a href="{% url 'profile_detail' %}">
-                <img src="{{ request.user.profile_picture }}" class="media-object"/>
+                <img src="{{ request.user.profile_picture }}" class="media-object float-left pr-2"/>
                 <div class="media-body">
-                    <b>{{ request.user.first_name }} {{ request.user.last_name }}</b>
-                    <p class="muted">{{ request.user.email }}</p>
+                        <b>{{ request.user.first_name }} {{ request.user.last_name }}</b>
+                        <p class="text-muted">{{ request.user.email }}</p>
                 </div>
             </a>
         </li>
-        <li class="mb-2">
-            <div class="custom-control custom-switch">
-                <input type="checkbox" class="custom-control-input" id="darkSwitch" />
-                <label class="custom-control-label" for="darkSwitch">Dark Mode</label>
-            </div>
-        </li>
+        <div class="dropdown-divider"></div>
         <li class="float-right"><a href="{% url 'logout' %}" class="btn btn-primary"><i class="fas fa-sign-out-alt"></i> Logout</a></li>
     </ul>
 {% else %}