VID-1990 Don't launch gallery builder if new galleries enabled

Good question. This didn't actually skip anon users, but forced them to login and then reloaded the page. UserLogin is [loaded on every page](https://github.com/Wikia/app/blob/dev/extensions/wikia/AssetsManager/config.php#L168), and if you look into `UserLogin.rteForceLogin();`, you'll see an [if clause](https://github.com/Wikia/app/blob/dev/extensions/wikia/UserLogin/js/UserLogin.js#L16) that always evaluates to true because wgComboAjaxLogin is [always set to false](https://github.com/Wikia/config/blob/dev/CommonSettings.php#L801). In that if clause the editor reloads.

I pulled the wgEnableMediaGalleryExt check into a second if clause because I thought it was more readable that way.