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!

RIGS/migrations/0040_profile_dark_theme.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.1.5 on 2021-02-06 10:43
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('RIGS', '0039_auto_20210123_1910'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='profile',
+            name='dark_theme',
+            field=models.BooleanField(default=False),
+        ),
+    ]