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

public/src/css/admin.css

@@ -107,10 +107,14 @@ main {
     width: 50px;
 }
 
-h1 small {
+h1 .song-id {
     color: #4a4a4a;
 }
 
+.song .song-id {
+    color: #a01300;
+}
+
 .form-field-indent {
     margin-left: 20px;
 }