Deprecate config.py for non-python format

[heartsucker]

Description

(This may be a duplicate)

Reporting this here while documenting TODOs (#724).

From install_files/ansible-base/roles/app/tasks/initialize_securedrop_app.yml:

# config.py.example is already written using Jinja2 format, and should be
# easy to template-ize. However, we cannot do this because when Ansible writes
# a template, it re-does the entire thing. This would cause a problem in the
# case where we want to re-provision a machine with an updated value (e.g. the
# application key fingerprint). Blindly overwriting the entire file would cause
# major problems. It would be nice to use templates here, assuming we find a
# way to selectively update the file.

Noting that part of the issue here is that we can't read values out of Python formats (easily) which makes it hard to apply differential updates (which is why this was reported originally).