Add the ability to subscribe via Preferred Address.

TODOS:

• Show some marker in the List options page that a user is subscribed via their Primary Address
• There is no way to know if a Member is subscribed via their Address or User to list Owners.
• There is no way to know in a subscription request page if it is for an address or a User.
• List summary page shows you are subscribed via some address, but it doesn't tell anything about whether you are subscribed as a user or as an address. Maybe that distinction doesn't need to be made either?
• Change subscription form will never show that one is subscribed via primary address.

Some screenshots: Subscribe Page:

After Subscribed:

Options Page for User: