Update panels.html to properly send data for nicknames

panels.html was sending the variable controlling nickname setting permissions without the data- tag, which core_chat_footer.js was expecting. Sending it without that tag caused the variable to always be null, and the button would then never subscribe to the event. Users were unable to change their nicknames due to this.

Testing:

Booted up an existing vm and changed allow-nickname-change to data-allow-nickname-change in Defaultv3's panels.html. Refreshed my browser and was able to change nickname when not logged in.