Improve how muting the chat is handled

Previously the chat looked for the presence of a cookie to decide if the chat was muted. This meant that muting was a global action: muting one chat mutes every other chat.

Now muting or unmuting does two things: it updates the cookie and a local variable. The chat looks at the local variable when deciding if the chat is muted. This means that muting is a local action with some additional global state.

So, as before, when you open a new chat window the mute setting will be as you left it last time and it is possible to open two chats and mute only one of them.

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It is necessary to modify the onSoundControlClick() to make this work. However, this repository only has a minified version of this function. I am pretty sure that the original needs to be modified as well, so here it is:

function onSoundControlClick() {
    let control = $('#chat-sound-control');
    if (control.hasClass('checked')) {
        chatmuted = true;
        setCookie('chatmuted', '1', 365);
        control[0].innerHTML = '<span class="icon"><i class="fas fa-volume-up" aria-hidden="true"></i></span>Mute Chat';
    } else {
        chatmuted = false;
        deleteCookie('chatmuted');
        control[0].innerHTML = '<span class="icon"><i class="fas fa-volume-mute" aria-hidden="true"></i></span>Unmute Chat';
    }
    control.toggleClass('checked');
}