Add favorite and retweet to young account blocking feature

[jsha]

From this user: http://go.telmina.com/2015/02/01/twitter-i-hope-for-appearance-of-anti-favorite-spam-service/, there is a request to add auto-blocking of young accounts on faves and retweets. It sounds like this is mainly to combat spambots rather than harassers.

If we do this, we'd probably want to roll it into the existing settings, to minimize settings bloat.

Pros:

• Covers more ways that a young account can try to get attention.

Cons:

• A little more complicated to explain on first sign up.
• Not necessarily related to harassment.