Link on Welcome email should pre-populate activation email

[mhudson]

The welcome_email view template has a link to "Activate your Swapsea account now".

However, a better UX would be to pre-populate the member's email, since they've come from the link in their the Welcome email - so we know it works.

Specifically, the /activate link should be /activate?email=<email>

(We don't give them a set password link immediately as it expires in 2 days and we don't want to handle unnecessary support calls.)