This lets us have a slightly cleaner interface for cases where we need
a custom default value than an `or None` in the definition (which
might cause issues with other falsey values).
This is the typical configuration in /etc/zulip/settings.py for a site
using S3, since there's no obligation to set the setting to None
directly, and we're not importing the main `settings.py` (and thus not
getting DEFAULT_SETTINGS applied).
Previously, thumbor.conf read LOCAL_UPLOADS_DIR from
zproject/settings.py by calling get-django-setting, which ends up
importing Django (taking 250ms, and requiring a full Zulip virtualenv
to be installed on the system).
The new system (in production) just imports prod_settings.py, which
has a lot fewer dependencies and is also a lot faster.
We need to use the value of SECURITY_KEY read from
/etc/zulip/zulip-secrets.conf (using get_secret), and also we disable
a few Thumbor features that we're not using. We also fix access to
secret keys by getting their values using get_secret.
For now, this does nothing in a production environment, but it should
simplify the process of doing testing on the Thumbor implementation,
by integrating a lot of dependency management logic.
Tim Abbott
Aditya Bansal