The email was branded "La Suite Numérique",
we updated the template to make it generic, we
will use settings env variables to customize the
email for each brand.
Change the email invitation content. More
document related variables are added.
To benefit of the document inheritance, we moved
the function email_invitation to the document model.
We want to be able to share a document with a person even if this person
does not have an account in impress yet.
This code is ported from https://github.com/numerique-gouv/people.
