✨(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/core/migrations/0002_alter_user_language_invitation_and_more.py

@@ -0,0 +1,42 @@
+# Generated by Django 5.0.3 on 2024-05-12 19:02
+
+import django.db.models.deletion
+import uuid
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='user',
+            name='language',
+            field=models.CharField(choices="(('en-us', 'English'), ('fr-fr', 'French'))", default='en-us', help_text='The language in which the user wants to see the interface.', max_length=10, verbose_name='language'),
+        ),
+        migrations.CreateModel(
+            name='Invitation',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, help_text='primary key for the record as UUID', primary_key=True, serialize=False, verbose_name='id')),
+                ('created_at', models.DateTimeField(auto_now_add=True, help_text='date and time at which a record was created', verbose_name='created on')),
+                ('updated_at', models.DateTimeField(auto_now=True, help_text='date and time at which a record was last updated', verbose_name='updated on')),
+                ('email', models.EmailField(max_length=254, verbose_name='email address')),
+                ('role', models.CharField(choices=[('member', 'Member'), ('administrator', 'Administrator'), ('owner', 'Owner')], default='member', max_length=20)),
+                ('document', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='invitations', to='core.document')),
+                ('issuer', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='invitations', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'verbose_name': 'Document invitation',
+                'verbose_name_plural': 'Document invitations',
+                'db_table': 'impress_invitation',
+            },
+        ),
+        migrations.AddConstraint(
+            model_name='invitation',
+            constraint=models.UniqueConstraint(fields=('email', 'document'), name='email_and_document_unique_together'),
+        ),
+    ]