Welcome email

Added a "Welcome to Loomio" mailer that is triggered by group setup completion 

Create example content before admin is added to group, so they are not sent email notifications about the new discussion and new proposal.

Also pulled out the erroneous text from the Sign In page (we can take a fresh look at this content when we we develop single sign-on).

Let's see what Travis has to say about this and then we can review and QA.