Support for custom analytics tracking vars on newsletters

 * Add ini flag to turn on and off MailChimp's automatic google analytics tracking of links.
 * If automatic analytics tracking is turned off, and some new ini settings are set, use them to build a UTM string, and append it to all links in the HTML copy of an email.
 * Add admin tool ability to set a custom tracking var as part of the UTM_Campaign.
I'm guessing that this should be ```analytics_utm_medium```. We already know that ```analytics_utm_source``` is not empty.