Move 'thanks for signing up' message into flash

I noticed the signups/:id page says "Thanks! We'll drop you a line when we launch!"—even when someone goes back to the page to check on how many referrals they've signed up.

This commit moves that into a custom flash message, so it only appears after an actual signup is completed.

Note: I used the custom key, flash[:signup_success], so as not to insert flash notices into application layouts somewhere else. Hopefully this will be pretty robust.