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_song_new.html

@@ -116,6 +116,11 @@
             </select>
         </div>
 
+        <div class="form-field">
+            <p><label for="lyrics">Lyrics</label></p>
+            <span class="checkbox"><input type="checkbox" name="lyrics" id="lyrics"><label for="lyrics"> Enabled</label></span>
+        </div>
+
         <button type="submit" class="save-song">Save</button>
     </form>
 </div>