Added mailing list info/sign up to contributing page, and some clean up.

Specifically, I merged the Report Bugs and Suggest Improvements into the Create
and new Discuss section. I removed stuff about creating new slides and audio
because that's no longer relevant.

Additionally, I added a Discuss section with links to the blog, announcement
mailing list, and the discussion mailing list. There is a form on the page
now that lets people (un)subscribe from the announcement mailing list.