configurable blog link

Adds a link to the header that leads to a blog. The url is in the configuration so we can do staging/production easily, or leave it blank so it doesn't appear at all.
On Fri, Oct 31, 2014 at 08:24:48AM -0700, Brian Sadler wrote:
> Huh, I didn't realize until now that we were adding config values that aren't actually secret in here.  It's a little weird, but I guess it's nice to just maintain all the configs in one big ENV var file for now?

Environment variables are the recommended way to do anything that differs
on staging and production with heroku because otherwise staging and production
are identical - they both are considered "production" deploys on Heroku and
the code comes off the same branch!