Fix file permissions

[willdurand]

Fixes mozilla/addons#16102

---

With this PR, we do not have any regular file with the wrong permissions:

$ find . -type f -not -perm 644 | grep -v -e ".sh$" -e "/scripts/" -e "manage.py" -e "/locale/" | wc -l
  0

FTR these files have different permissions on purpose:

$ find . -type f -not -perm 644
./docker/entrypoint.sh
./locale/generate_category_po_files.py
./locale/stats-po.sh
./docs/build-github.zsh
./scripts/check_debian_packages.sh
./scripts/update_assets.py
./scripts/install_deps.py
./scripts/run_l10n_extraction.sh
./scripts/generate_build.py
./scripts/setup.py
./scripts/health_check.py
./scripts/compile_locales.py
./scripts/check_pip_packages.sh
./scripts/health_check_blocks.py
./scripts/sync_host_files.py
./manage.py