[#1996] Fix featured group link.

URL was previously set as a jinja template variable, but was being
called inside a child block of the template, so the var was out of
scope and coming up empty. See:
http://jinja.pocoo.org/docs/dev/templates/#block-nesting-and-scope
Merge pull request #1996 from brew/1996-featured-group-link

Featured group on homepage not linking to group