Add a new trigger to reminders based on editorial metadata of type Date

[andergmartins]

This is the user's request:

Is it possible to send reminders based on the First Draft Due Date metadata? We'd like to:

1. send authors reminders three before the draft is due,
2. send authors and the managing editor notices on the day the draft is due.

We can add two new options to the "When to notify" column:

[andergmartins]

Related to #567