Notifications on reply to a thread

[mjumbewu]

Feature description

User should be able to receive email notifications upon a new comment on a place.

This would require changes here and in https://github.com/openplans/shareabouts-api/. Need to allow for unsubscribing from notifications too. Start simple as possible

• Two-step subscription, with a customizable email confirmation
• Authentication-less unsubscribe view with simple confirmation

Steps to consider

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This feature is done when

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