First stab at per-hunt settings

[rgossiaux]

This commit

• adds a HuntSettings model which contains all the settings for a hunt.
• initializes HuntSettings for existing Hunts using the current values for environmental variables in your environment
• creates a new HuntSettings object when a Hunt is created
• uses the HuntSettings preferentially for Google-related tasks (the human Drive folder, creating new sheets), falling back to env variables if not set
• exposes the HuntSettings in the Django admin

Can follow-up with a change to use HuntSettings for Discord-related tasks as well.

For supporting multiple independent users, after that we still need to:

• expose a way to set/update these in the UI
• do all the permissions stuff

which there is definitely not time for this year.

Concretely this will at least let us change these settings during hunt if for some reason we need to, by using the django admin.