Use Django default file mode-settings for uploaded files, and limit use of system()

[johnbergvall]

These commits make the following changes:

• Use context manager to make sure file handlers are closed directly after use
• Use Django settings FILE_UPLOAD_PERMISSIONS and FILE_UPLOAD_DIRECTORY_PERMISSIONS as fallback to determinate new file- and directory modes. This also makes sure to use correct permissions for all created subdirectories.
• Use shutil.chown() and os.chmod() instead of spawning separate shell command to recursively change file permissions

The shutil.chown() and os.chmod() also fixes an issue where the previous recursive command may set incompatible permissions on any subdirectories for the path (e.g. no +x-permissions)

In case you would want me to split the pull request, using settings.FILE_UPLOAD_PERMISSIONS will need separate handling to not overwrite directory permissions due to the recursive flag in the system-command.