merge social links into a "contact us" page

[FromAnkyra]

as @LukeMoll mentioned in #164 , the navbar is getting reasonably crowded, and one way to reduce that would be to have all our social media links in one page, instead of links taking us directly to them on the navbar

[LukeMoll]

Also worth noting, some of this information is already present on about.html, if it's broken out into a new heading then we could add all the links in one place.