Fix isDeletable

- Fixes default `isDeletable` to check if the parent is writable
- Don't start recursing into a directory when it's not deletable

To test:
- on master:
- Create a folder and put some files in there
- Outside oc, change the owner of the folder so the web server no longer has write access to it (only the folder, not the files inside)
- rescan the filesystem
- Try to delete one of the files inside the read-only folder (which are shown as deletable in the web-uid)
- Switch to this PR
- Rescan the filesystem
- The files inside the read-only folder should now show as non-deletable

cc @PVince81 @DeepDiver1975