fix: prevent sharing permissions on user root folder

[skjnldsv]

A user root folder is not shareable.

server/lib/private/Share20/Manager.php

Lines 274 to 282 in 825b65e

	// And you cannot share your rootfolder
	if ($this->userManager->userExists($share->getSharedBy())) {
	$userFolder = $this->rootFolder->getUserFolder($share->getSharedBy());
	} else {
	$userFolder = $this->rootFolder->getUserFolder($share->getShareOwner());
	}
	if ($userFolder->getId() === $share->getNode()->getId()) {
	throw new \InvalidArgumentException('You cannot share your root folder');
	}

[skjnldsv]

/backport to stable27

[skjnldsv]

/backport to stable26