Add custom Don

- Registered users can customise the colour of their Don and it will appear for other players
- Bug fixes:
  - Add lyrics checkbox to admin page
  - 2P shows above "creative" or "with lyrics" labels
  - Prevent accidental alt and menu keyboard presses from triggering browser menus
  - Fixed mouse hitboxes on difficulty selection
  - Clean cached sounds and lyrics when another song is loading
  - Fixed debug jumping to the top-left of the screen when hidden
  - Fixed server volume not being applied to songs

schema.py

@@ -34,6 +34,15 @@ update_display_name = {
     }
 }
 
+update_don = {
+    '$schema': 'http://json-schema.org/schema#',
+    'type': 'object',
+    'properties': {
+        'body_fill': {'type': 'string'},
+        'face_fill': {'type': 'string'}
+    }
+}
+
 update_password = {
     '$schema': 'http://json-schema.org/schema#',
     'type': 'object',