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!

pipeline/source_assets/scss/dark_screen.scss

@@ -111,4 +111,12 @@
     .custom-control-input:focus ~ .custom-control-label::before {
         box-shadow: 0 0 0 $input-focus-width rgba($primary, 0.7) !important;
     }
+    .source {
+        color: white !important;
+    }
+    .embed_container {
+        border-color: #3853a4 !important;
+        background: #222;
+        color: $gray-100;
+    }
 }

pipeline/source_assets/scss/screen.scss

@@ -1,4 +1,3 @@
-@import "dark_screen";
 @import "custom-variables";
 //Required
 @import "node_modules/bootstrap/scss/bootstrap-reboot";