Compatibility with bedrock-autoloader which loads plugins as must-use plugins

[gerwinjansen]

Fixes 2 problems:

• Couple of times plugins_url() . '/' . plugin_basename( dirname( __FILE__ ) ) . '<relative url here>' was used which resolved to /plugins/wp-youtube-lyte/<relative url here>. And that doesn't work when the plugin is loaded as /mu-plugins/wp-youtube-lyte.
• As workaround for issue roots/bedrock-autoloader#29, I explicitly made $lyteSettings declared as global.