Add include mailinglist form.

https://github.com/madewithbytes/us_ignite/issues/163
Update wording on the submit button.
Include mailing list form in the blog pages.

https://github.com/madewithbytes/us_ignite/issues/163
Redirect to the homepage after successful mailing list registration.

https://github.com/madewithbytes/us_ignite/issues/163

Doesn't add any value to redirect to the subscription form after
being registered.
Include mailing list form in the homepage.