Notifications via email aka subscribe/unsubscribe, aka watch/unwatch

[ctrager]

The UI:

global enable in budoco_settings_active.txt for notifications

(global enable whether to send notifications to email addresses that are not users (from incoming emails) - but then we'd need a way for external users to unsubscribe, so.... no, don't do this)

at user, radio buttons
subscribe to all
subscribe only to issues created/updated by me <<== default
subscribe to none

new user_issue table for explicit sub/unsub

The Backend:

We want to handle where user updates an issue, then quickly updates it again, and again. We want to send only one notification. So when we queue up the notifications, don't do multiple, and make sure they are stale by a configurable amount of minutes in settings file.

For now, email doesn't have to say what was updated.

Don't send an email to the person who did the update.