Allow users to opt-out from login alerts

[ChristophWurst]

I can imagine that not all users care about these events and might want to disable the notifications for their account.

There is also the case where a user has clients connected that use the login password. In that case the classifier might very often warn about a login, though there was none. For flawless security we can't duck type a connection to being a client because attackers can also just use a Nextcloud client to access the victim's data. Hence the separation of web login and client access does not help here.

The proposed solution is a simple checkbox on the user's personal settings page that default to checked. If unchecked, the app will never send out notifications.

cc @rullzer as discussed