Add a configuration option to define the permissions of created file system directories

When papaya creates the media and cache directory structue they are created readable, writeable and executeable for all (777, rwxrwxrwx).
When saving a media file it is created readable and writeable for all (666, rw-rw-rw-). Cache files are created readable and writeable for the owner and readable for group and others (644, rw-r--r--).

In some cases 600/700 should be enough, so only the owner (the user who runs the webserver; www-data per default on a debian/ubuntu server) can open the directories and read/write the files.