Set fixed position for notifications

[Erik Michelson]

Description
When performing an action that produces a notification and you're scrolled down, you won't see the notification because notifications are rendered always relative to the document top position.

To Reproduce

1. Go to https://hedgedoc.dev/profile
2. Scroll down to "Account management" and try to delete your account (wait the 10 seconds in the modal).
3. The notification is not in the visible viewport (unless you're using a monitor in portrait orientation).
4. Scroll up to the top of the page and see the notification.

Expected behavior
See the notification always in the top of the viewport no matter how far you've scrolled.

Additional context
position: fixed instead of (inherited) position: relative