Remove html tags from strings that may appear in notifications

Remove html tags from strings that may appear in notifications

The content of notifications on MacOS are not allowed to contain HTML
tags in it. Linux (Free Desktop) notifications may contain HTML tags
in their content. However, it is optional that the notification daemon
interprets the tags. The GNOME notification daemon does not interpret
HTML tags and even does not strip them off. KDE's daemon interprets
the tags. For now I think it's the best solution to remove the HTML
tags. In the future we should fire a action if the user clicks on the
notification and react on it and open the link then. This should work
on all platforms. But as this change involves working with the native
MacOS api and I don't have hardware for that at the moment for that I
will postpone it.

Fixes #2200

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>