Make RSS feed valid

/CC @Underground78 @JellyFrog

Basically, we need to remove any `class`, `data-foo`, `iframe` and all [these](http://validator.w3.org/feed/docs/warning/SecurityRisk.html).

With this patch  http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fmpc-hc.org%2Frss.xml