Email notification on issue due date

What does this MR do?

Every day, sends an email for each issue that is due tomorrow, to all participants and subscribers, as well as people with the custom notification level set.

Todos for Sean

• Fix broken specs.
• Ensure this only happens for open issues.
• Check / update docs.

Are there points in the code the reviewer needs to double check?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16957#note_57883273 is probably the most fiddly part, but I think it's OK for now.

Why was this MR needed?

It's a nice feature 🙂

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/27500.