option to notify when next card becomes due

[dae]

Moving this from my private list of issues:

For users who set their learn ahead limit to 0, we could be scheduling a notification when the first learning card (or some threshold of them) becomes due. That does seem useful, and is already on the todo list.

https://forums.ankiweb.net/t/functional-notifications-are-essential-for-ankimobile/66148/4