optionally motivate users to join the site

Issue Summary

This module's motivation is to keep people engaged, but several of its features are less useful if the user doesn't have a full account.

For example, a user who is registered can potentially unsubscribe from all of their subscriptions because they have proved that they own the account. They can also change their e-mail address on a site wide basis.

So, how do we motivate them?

• We could add a token to the notification mail for "create an account"
• We could have an easier way to create users that prefills the e-mail because we've already got that (that may belong in a different module)
• If email registration is enabled.