Shared Drive scanning doesn't record whether the scan was successful or not

Description

The shared drive scanning operation is using last_updated for when the drive scan was last attempted rather than when it was last successfully scanned.

This means the shared drive usage summary incorrectly reports usage and managers from the last successful scan but with a last updates ("as of") of the last attempt.

Further details

Separating the last scan attempt (needed to avoid repeatedly scanning the same drives in the next batch) from the actual last successful update of the usage and permission data, would not only correct the misleading date in the usage reported but would allow us to identify drives that we no longer are able to scan (last successful update would get old).

Task list

• Use last_updated for successful update
• Add last_scan_attempt for when it was attempted (which may match last_updated if successful)
• Use last_scan_attempt when determining which batch of shared drives to do next

Acceptance criteria

• Shared drive usage summary no longer mislead when the usage and manager data was updated
• shared_drive_list.yaml contains data on when the drive was last successfully scanned as well as last attempt