Make the unauthenticated homepage more engaging

[timothyfcook]

Since the "top ten lists" have been taken off the homepage, we need some way to get potential users excited about contributing. They can't do this unless they see examples of what they could support.

We need to tell the stories of users and highlight certain profiles on the homepage.

Communities should also be available for browsing.

Basically, we need something beyond search on the homepage.

[Changaco]

#2584 is about the homepage for signed-in users, let's use this issue for ideas about improving the homepage viewed by unauthenticated users.

[timothyfcook]

Here's an article that can help inspire discussion: http://www.helpscout.net/blog/user-onboarding-mistakes/

[chadwhitacre]

Done in #3718, etc.

[chadwhitacre]