feat(files_external): Enable ownerless shares

[provokateurin]

Summary

Simple follow-up to #49073.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[come-nc]

@provokateurin A comment explaining what the class is would help.
SystemMountPoint is for mounts configured by admin and available for several users, while PersonalMount is for mounts added by user themselves in their tree?
What about class ExternalMountPoint, it should be abstract, no?

[provokateurin]

SystemMountPoint is for mounts configured by admin and available for several users, while PersonalMount is for mounts added by user themselves in their tree?

Yes

What about class ExternalMountPoint, it should be abstract, no?

It implements some of the methods, so I think this is how it should be 🤷‍♀️