Replace the existing explicit environment-variable based loading of
secrets with the new externalsettings method. Rather than setting a load
of environment variables in the development containers, we add a
settings document inside the container and specify its location via an
EXTRA_SETTINGS_URLS environment variable.

The means that the same mechanism to load settings in development is
used in production and so failure to add settings to
{required,optional}_settings will be caught early.

Tox has been updated to support the new way of setting values.