In #98 some of the challenges with having renovate keep dependencies
fresh in this repo were discussed. In particular the contents of the
template/ directory are, well, templated and so there's no guarantee
that renovatebot will be able to parse them. Even if we made them
parseable, it's hard to enforce continued parse-ability in CI.

Instead, lean in to the templating approach by adding files which
renovatebot *can* parse specify the minimum version requirements and
have copier read from that file to populate the requirements within the
template.

Fortunately the copier installed alongside renovatebot has support for
copier-templates-extensions installed which, in turn, allows adding
custom extensions to the template context via Python scripts inside the
template repo.

Combine both things together: we now have `template*requirements.txt`
files which renovatebot *can* parse and update which are used to
replace constructions such as `{{ deps.somepackagename.version }}` in
the `pyproject.toml` and `.gitlab/webapp.gitlab-ci.yml` files. When
renovatebot updates the `template*requirements.txt` files, the templates
will automatically make use of the new minimum versions.

Note that these are always *minimum* versions. We don't want to stop
renovate from being able to _additionally_ update dependencies in
templated repos even if a new version of the template with bumped
dependencies hasn't been released.

Add some configuration to `renovate.json` to reduce the number of MRs
likely to be raised by renovate to a manageable rate.

[1] https://github.com/renovatebot/renovate/discussions/34368