Send email digest

[dvassallo]

Send a weekly email digest to all members with the top 10 topics of the previous week.

• Same topic (based on ID) should not appear in multiple emails.
• The topics should be ordered by the same ranking algorithm as we have in the home feed, filtered by time (previous week).
• The topics in the email should link to the actual topic in campfire.
• In the profile page add a new option to receive weekly email digests (on by default): https://smallbets.com/users/me/profile.
• Add an unsubscribe link in the email that automatically turns off that notification.

Nice to have:

• Control this feature from the Admin section (/account/edit).

WIP by @renatonitta