[BUG] Notifications appear also on cancelled lectures

[mipro98]

Describe the bug
Notifications about the upcoming lecture are also sent even though the lecture has been cancelled in TUMOnline.

To Reproduce
Steps to reproduce the behavior:

1. Make sure to have notifications for next lecture enabled in the settings
2. Have an upcoming lecture which has been cancelled (which is also clearly shown in calendar view)
3. About one hour before the lecture, notifications are sent from the app to remind you about the lecture

Expected behavior
A notification should not be shown if a lecture has been cancelled.

[kunzef]

Hi just wanted to let everyone know that I'm currently working on #1235 and once finished, would with the gained knowledge there, tackle this issue.