Minor documentation/configuration issue regarding signatures.zip location

There is a documentation or configuration issue that some folks including me ran into when trying to set up funfuzz for the first time (see MozillaSecurity/funfuzz#159). I'll repost the description of the problem anyways:

The FuzzManager readme somewhere describes a way to update the signatures.zip file using a cron job like this:

# Export all signatures to a zip file for downloading by clients
*/30 * * * * cd /path/to/FuzzManager/server && cronic python manage.py export_signatures files/signatures.new.zip mv files/signatures.new.zip files/signatures.zip

However SIGNATURE_STORAGE which is used in the SignaturesDownloadView is defined as follows in server/server/settings.py:

# This is the directory where signatures.zip will be stored
SIGNATURE_STORAGE = os.path.join(BASE_DIR)

On my setup this equals to server/ and not server/files. The problem was solved after changing the config to:

SIGNATURE_STORAGE = os.path.join(BASE_DIR, 'files')

Putting signatures.zip into BASE_DIR should also fix this w/o the need to update settings.py.

So either way FuzzManager readme should be updated to reflect the actual settings in server/server/settings.py or server/server/settings.py should be updated to reflect the documentation in the readme.

Compare the FuzzManager README's cron command with SIGNATURE_STORAGE in server/server/settings.py and the SignaturesDownloadView. Decide whether the documented path or the setting should be authoritative, update the relevant documentation or configuration for a consistent signatures.zip location, and verify that the setup and download flow use that location.