Move to generating image build jobs for Django, Logan/terraform and
Python dynamically by querying the appropriate upstream APIs.

This is mostly a genericisation of the previous logan terraform work. We
extend our version parsing code to understand PEP440-style versions and
code in some knowledge about which Django versions are compatible with
which Python versions.

Aside from a fair amount of Python code, the only _changes_ to image
building are as follows:

* The Python alpine image has been updated to allow passing Django
  versions dynamically rather than having a
  `requirements/django-{...}.txt` file for each Django release.
* We've had to put in some horrible hackery to support _both_ envsubst
  and build args for Docker image building.
* Building of django, Python and logan/terraform images are serialised
  by means of a `resource_group` since, in testing, the number of
  dynamic jobs could easily overwhelm the VM runners.

Anticipated Python version support changes for Django 5.0 have been
coded in so that we don't get taken by surprise on Django 5.0 release
day.