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-lock.json

@@ -18,7 +18,6 @@
         "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",
@@ -1663,11 +1662,6 @@
         "type": "^1.0.1"
       }
     },
-    "node_modules/dark-mode-switch": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/dark-mode-switch/-/dark-mode-switch-1.0.0.tgz",
-      "integrity": "sha512-wRoqYGmph7mwRbPcSNRR6DpRQe5wOK0lG6O3Hz7nIWWDyLAHSqje7PUI6c/acOYACiSPHc7sqysn57GSuklb6w=="
-    },
     "node_modules/dashdash": {
       "version": "1.14.1",
       "resolved": "https://registry.npmjs.org/dashdash/-/dashdash-1.14.1.tgz",
@@ -10583,11 +10577,6 @@
         "type": "^1.0.1"
       }
     },
-    "dark-mode-switch": {
-      "version": "1.0.0",
-      "resolved": "https://registry.npmjs.org/dark-mode-switch/-/dark-mode-switch-1.0.0.tgz",
-      "integrity": "sha512-wRoqYGmph7mwRbPcSNRR6DpRQe5wOK0lG6O3Hz7nIWWDyLAHSqje7PUI6c/acOYACiSPHc7sqysn57GSuklb6w=="
-    },
     "dashdash": {
       "version": "1.14.1",
       "resolved": "https://registry.npmjs.org/dashdash/-/dashdash-1.14.1.tgz",