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

templates/admin_songs.html

@@ -11,9 +11,9 @@
     <a href="/admin/songs/{{ song.id }}" class="song-link">
         <div class="song">
             {% if song.title_lang.en %}
-                <p>{{ song.title_lang.en }} <small>({{ song.title }})</small></p>
+                <p><span class="song-id">{{ song.id }}.</span> {{ song.title_lang.en }} <small>({{ song.title }})</small></p>
             {% else %}
-                <p>{{ song.title }}</p>
+                <p><span class="song-id">{{ song.id }}.</span> {{ song.title }}</p>
             {% endif %}
         </div>
     </a>