Admin feature: disable/configure mount options for personal storages

[PVince81]

Currently in 8.1 (master) the external storage mount options (#12216) are available for both global mounts (admin page) and also for personal/user mounts (personal page).

It could be useful if the administrator was able to decide which mount options a user is allowed to use/set on their personal storages. For example an admin might not want a user to be able to disable encryption or to set the watched policy to "Always".

@MTRichards @msrex @Xenopathic @icewind1991 @DeepDiver1975