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!

assets/models.py

@@ -146,10 +146,7 @@ class Asset(models.Model, RevisionMixin):
         return reverse('asset_detail', kwargs={'pk': self.asset_id})
 
     def __str__(self):
-        out = "{} | {}".format(self.asset_id, self.description)
-        if self.is_cable and self.cable_type is not None:
-            out += '{} - {}m - {}'.format(self.cable_type.plug, self.length, self.cable_type.socket)
-        return out
+        return "{} | {}".format(self.asset_id, self.description)
 
     def clean(self):
         errdict = {}