Housekeeping: Set clearing old notifications after a time

[teamcons]

Problem

The backend part of this FR: elementary/switchboard-plug-security-privacy#179

Housekeeping does not allow to delete old notifications, so users end up with notifications and informations from weeks ago if not cleared manually regularly
if cache, trash, are cleared automatically, it makes sense for notifications to follow a housekeeping schedule

Proposal

Enable housekeeping to clear too-old notifications, according to a setting on how old they are allowed to be, which would be set in Privacy/housekeeping

Prior Art (Optional)

No response