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!

package.json

@@ -14,7 +14,6 @@
     "bootstrap-select": "^1.13.17",
     "clipboard": "^2.0.6",
     "cssnano": "^4.1.10",
-    "dark-mode-switch": "^1.0.0",
     "flatpickr": "^4.6.6",
     "fullcalendar": "^5.3.2",
     "gulp": "^4.0.2",