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/screen.scss

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