Remove email invitation from Invitation model
to be able to use it in other context.
We add it in utils.py instead, and it will be called
from the viewset.
We add the document_id to link to the document from
the mail.
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.
