Enforce existance of /files/ inside object store home folder

**Steps to reproduce:**

1. Use ObjectStore as primary storage
2. delete from oc_filecache where storage = 1; // 1 == admin home
   storage
3. Try to login or go to the files app => breaks
4. This doesn't happens on a local storage

Also add a minor optimization in ObjectStoreStorage since mkdir already
checks if file exists before creating a folder.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>