Abort scanning users with a large number of files/folders and mark for manual attention

Some users have over a million files which cannot be scanned before the CI timeout. The schedule then gets stuck repeatedly attempting to scan the same user.

• Add a configuration option for maximum user scan duration (beware confusion with max_scan_duration which is total "scan" operation time - this may need renaming).
• Make the file/folder listing part of the "scan" operation abort after this duration
• These users should have their ucam schema mydrive action prefixed with "manual-" so they are no longer included in the automatic scanning
• (Optional) It may be good for the "scan" operation to give a big warning if it sees any "manual-*" users, even though it won't process them