Notifications: Create e-mail metrics display for admins

[PaRangger]

Is your feature request related to a problem?

It is currently challenging to identify issues with email delivery. Administrators can only determine if email sending is failing by reviewing the application logs.

Describe the solution you'd like

A dashboard feature in the admin metrics overview displaying the number of emails successfully sent and failed over the past few days.

Describe alternatives you've considered

An admin-only notification if e-mails are failing. However that may need defining a threshold for failed e-mails in a certain timeframe, since sometimes mails might just fail for unproblematic reasons.

[krusche]

Regarding announcements, I could also imagine a progress bar (similar to the preparation of student exams) based on websockets. Those announcements are written in a popup. Those could include an indication how many students would be notified.

When the instructor clicks on Send, the progress bar starts updating. In case the instructor is not interested (e.g. because it takes a while to notify 2000 students), she/he could also close the popup, but the email sending would still continue.

In case some emails could not be sent, additional information can be opened, e.g. a list of students / emails for which sending did not work.