Fix poetry warnings

As noted in https://gitlab.developers.cam.ac.uk/uis/devops/regent-house-ballots/ballots-webapp/-/merge_requests/14:

Running poetry install with later (>= 1.8) versions of poetry results in the following dire warning:

Warning: The current project could not be installed: No file/folder found for package ucam-card-api
If you do not want to install the current project use --no-root.
If you want to use Poetry only for dependency management but not for packaging, you can disable package mode by setting package-mode = false in your pyproject.toml file.
In a future version of Poetry this warning will become an error!

Heed the warning and add package-mode = False into pyproject.toml. This is correct because we use poetry only for dependency management in our webapps; we do not use it to actually build or distribute any packages.

This commit should be back-ported to the boilerplate repo.

Back-port that change to the boilerplate.