New option to disable sharing on external storage mounts

[PVince81]

Ref: owncloud/core#19157

If any shares existed already on that storage, they will continue to exist internally but will not be accessible for recipients any more. In the event where the admin will re-enable sharing for such mounts, the shares will become accessible again.

@carlaschroder

CC @icewind1991 @jmaciasportela

[PVince81]

Additional note: this disables sharing completely. So both user shares and public shares become unavailable.

The original ticket mentioned splitting these two, but it isn't implemented yet. 9.0 will have a single option that disables both.

[carlaschroder]

Thank you @PVince81!

[carlaschroder]

e48aec5