Implement Due Dates notification emails

[julen]

We need to support the following types of notification emails:

1. Notification that a new task has appeared in the top 3 positions of the priority list. This notification should not be triggered when the top priority task is completed and the other ones naturally move up.
2. Daily digest (what to work on today).

Subscription logic

• Auto-subscription: Once the translator submits a translation for any language, we want to subscribe them for notifications for that particular language.
• Opting in/out: They should be able to opt-in and opt-out of notifications for any other languages. The fact that someone has opted out of notifications for a particular language should be remembered forever and take precedence over the abovementioned auto-subscription logic.
  Volunteers should be able to opt-in and opt-out of notifications for any language at any time. Once they have subscribed for language notifications, the "Subscribe to notifications" link at the bottom of the task dashboard can be replaced with: "You are subscribed to notifications in . Unsubscribe".

Emails

Any notification emails that we send there should be an "Unsubscribe from notifications for " link in the footer.

For "new task" notification emails, we might want to give a ~5 min grace delay before sending out a notification (to allow admins change the date if it was set incorrectly, for example). So if after 5 minutes there are no changes to the list, we send out notifications to translators.

Prerequisites

• Admin widget in browsing widget (Add Due Dates widget in browsing pages #4801)
• Browsing page dashboard (Implement Due Dates dashboard #4802)