[oc6] don't allow to create a file or folder named 'Shared' in the root folder

don't allow to create a file or folder named 'Shared' in the root folder, also exclude all combinations of lower and upper case letters

Backport of #7531 

Tested:

- [x] sync client created "Shared" or "shared" file inside root: forbidden
- [x] sync client created "Shared" or "shared" dir inside subdir: ok
- [x] sync client move "Shared" or "shared" to a different subdir: ok
- [x] sync client move "Shared" or "shared" from the subdir to the root: forbidden
- [x] sync client update file in existing "Shared" directory: ok
- [x] WebDAV create "Shared" or "shared" dir in root: forbidden
- [x] WebDAV create "Shared" or "shared" file in root: forbidden
- [x] WebDAV create "Shared" or "shared" dir in subdir: ok
- [x] Web UI create file/folder called "Shared" or "shared" in root: forbidden
- [x] Web UI create file/folder called "Shared" or "shared" in subdir: ok
- [x] Web UI upload file called "Shared" or "shared" in root: forbidden
- [x] Web UI upload file called "Shared" or "shared" in subdir: ok
- [x] Web UI: Move "Shared" from subdir to root: forbidden

cc @PVince81