Disallow sharing when using "Save in session" for ext storages

[PVince81]

When using ext storages with "User credentials, save in session", sharing cannot work because the credentials are not available when logged in as recipient user.

This is documented here: owncloud-archive/documentation#4283

Let's disable the option in the UI and also prevent it to be set from occ to avoid people wondering why sharing doesn't work.

[ownclouders]

GitMate.io thinks possibly related issues are #28636 (Ext storage checkbox to disallow sharing on user-created mounts), #8503 ("Possible share permissions" issue when sharing ext storage mount point), #9296 ([master] Share icon invasion when sharing multiple "Local" ext storages), #9340 (Disallow moving personal ext storage mount points to other mount points), and #5949 (Improving ext storage with streaming).