✨(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/pyproject.toml

@@ -42,6 +42,7 @@ dependencies = [
     "dockerflow==2022.8.0",
     "easy_thumbnails==2.8.5",
     "factory_boy==3.3.0",
+    "freezegun==1.5.0",
     "gunicorn==22.0.0",
     "jsonschema==4.20.0",
     "markdown==3.5.1",