Trigger user notification in case an infected file was found by background scanner

[PVince81]

Usually infected files are caught early and cannot be uploaded.

However, there are some scenarios where an infected file could find itself already on the storage:

• at the time of the upload, the signature database wasn't up to date or virus was not known yet
• someone uploaded the file directly to an external storage and bypassing OC

The background scanner which runs with cron is able to find these. Currently we only log an entry in the log about such infected files. (@VicDeo what else ?)

We should also provide a way to send a user notification using the notifications API.

• I'd say send one to the all admins at least.
• send to storage owner as well ?
• we can't send to original uploader as we don't track who uploaded what

As a reminder, such notificaction would appear under the bell icon in the web UI and possibly on desktop and mobile devices of said user.

@VicDeo does the antivirus scan external storages in the background ?

@pmaier1