🚸(backend) make document search on title accent-insensitive

This should work in both cases:
- search for "vélo" when the document title contains "velo"
- search for "velo" when the document title contains "vélo"

src/backend/core/migrations/0021_activate_unaccent_extension.py

@@ -0,0 +1,10 @@
+from django.contrib.postgres.operations import UnaccentExtension
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("core", "0020_remove_is_public_add_field_attachments_and_duplicated_from"),
+    ]
+
+    operations = [UnaccentExtension()]