Create invitation model, factory and related tests to prepare back-end
for invitation endpoints. We chose to use a separate dedicated model
for separation of concerns, see
https://github.com/numerique-gouv/people/issues/25
The admin was broken as we did not worry about it up to now. On the frontend
we want to use OIDC authentication only but for the admin, it is better if
the default authentication works as well. To allow this, we propose to add
an "email" field to the user model and make it the identifier in place of
the usual username. Some changes are necessary to make the "createsuperuser"
management command work.
We also had to fix the "oidc_user_getter" method to make it work with Keycloak.
Some tests were added to secure that everything works as expected.
Used https://github.com/openfun/joanie as boilerplate, ran a few
transformations with ChapGPT and adapted models and endpoints to
fit to my current vision of the project.
