(reapply) Redirect CMS help links to Stanford Google Group

cherry-picked from 12b79db6fc5dfd3ae9f3c558b151e004852eee7d

Move help links from `help.edge.edx.org` to the new studio-users
Google Group. Change references to `Studio Help` to the group as
well. Replace the `Contact Us` Tender widget with a `mailto` to
the Google Group email address.

Conflicts:
	cms/templates/login.html
	cms/templates/widgets/footer.html
	cms/templates/widgets/header.html
	cms/templates/widgets/sock.html