Fix Youtube user parsing for whitelisting script

var exposeUserInURL = function() {
    'use strict';

    // To fix issue with user name, parse it from a different query, so
    // the following method works currently. Variable channelId will thus
    // contain the Youtube user name.
    // Replace lines from "var link = document.querySelector ..." to
    // "var channelId = ..." with the following code (within "// ---"):
    // ---
    var link = document.querySelector('[id="watch7-user-header"] a[href^="/user/"]');
    if ( link === null ) {
       return;
    }
    var linkHref = link.getAttribute('href');
    var linkmatch = linkHref.match(/\/user\/(.+)/);
    if (linkmatch === null)
        return;
    var channelId = linkmatch[1];
    // ---

    // Code below need not be changed
    var newArg = channelId !== '' ? 'user=' + encodeURIComponent(channelId) : '';