Notification to release working times for approval

This would be a major feature and require a lot of work. The idea is to get a notification per Mail, Messenger, phone Notification, etc. that working times still have to be released for approval.

Implementation wise

Use a backend service which tracks per employee if, the last time the extension loaded working times, any working times were not released for approval. Credit to @Darquaise for the idea.(1)
Every time the extension loads working times, it checks for not released times. It sends the first date with unreleased working times and some employee identification (probably some hash).
The employee would register some way to get their notification (consider account management and privacy policy), via the extension pop-up, in the backend. The backend will then check at the end of the week and month if all times have been released for approval and send notifications to the according employees.

The complete feature should be opt-in. The goal should be to save a minimal amount of personal information in the backend.

(1) Older ideas included a backend which would have to log in to the employee's account (obviously not feasible) and then check if there are any working times not released for approval.

Start by mapping the browser extension popup and the proposed backend service described in the issue. Define the opt-in registration, minimal employee identification, unreleased-time tracking, and email, messenger, or phone notification flow; done means end-of-week and end-of-month checks notify the correct employees without storing unnecessary personal information.