Send email reminders

The users who requested this module also wanted to optionally send reminders to an email alias when possible translation activity occurs.

Possible notification settings:

1. Anytime a page is updated into non-neutral.
2. Anytime a non-neutral (source) page is edited (include: updater, when translation were created, diff).
3. Regular reports for pages marked with needs translation.
4. Regular reports for non-neutral pages with translations missing.
5. Regular reports for unpublished non-source translations.