✨(models/api) allow inviting external users to a document by their email

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.

src/backend/impress/settings.py

@@ -281,6 +281,7 @@ class Base(Configuration):
     EMAIL_FROM = values.Value("from@example.com")
 
     AUTH_USER_MODEL = "core.User"
+    INVITATION_VALIDITY_DURATION = 604800  # 7 days, in seconds
 
     # CORS
     CORS_ALLOW_CREDENTIALS = True