app/views/invitation_mailer/invite.html.erb
<!DOCTYPE html>
<html>
<head>
<meta content='text/html; charset=UTF-8' http-equiv='Content-Type' />
</head>
<body>
<h1>You have been invited to create a Census Account</h1>
<p>Census is the Turing School of Software Design's identity management application.</p>
<button type="button" name="button"><%= link_to "Register", @url %></button>
<p>Thanks for joining!</p>
</body>
</html>