fix: Repair invalid mountpoints to make partial mounts work

[provokateurin]

In

groupfolders/lib/Mount/MountProvider.php

Line 286 in ef29cca

$parts = explode('/', $mountProviderArg->mountInfo->getMountPoint());

we get a normalized path which may not match the stored mountpoint as queried in

groupfolders/lib/Folder/FolderManager.php

Line 675 in ef29cca

$query->andWhere($query->expr()->in('f.mount_point', $query->createParameter('path')));

due to backslashes for example.

We already prevent creating new Team folders with invalid mountpoints, but we need to make sure that all existing mountpoints are fixed in order for the queries to work as intended.

[provokateurin]

/backport to stable33