DatabaseStorage.path fail for files in the database but not on localstorage

[rhunwicks]

Currently DatabaseStorage.path is:

        localpath = self._path(self.get_instance_name(name))
        if not os.path.exists(localpath):
            raise NotImplementedError
        return localpath

I.e. if the file doesn't exist locally then it raises NotImplementedError.

However, it is possible that if there are multiple webservers in front of a single database, then the file may be in the database but not in the local storage. The existing _open includes:

                # Automatically write the file to the filesystem
                # if it's missing and exists in the database.
                # This happens if we're using multiple web servers connected
                # to a common database behind a load balancer.
                # One user might upload a file from one web server, and then
                # another might access if from another server.
                utils.write_file(name, f.content)

We should apply the same approach to path to automatically write the file to the local filesystem if it exists in the database but not locally.

[rhunwicks]

@jeverling for info.