fix(import_report): display "Données importées" panel only when done

[VincentCauchois]

Display panel "Données importées" only when import is "done", not when "in progress" or "in error".

How to test

1. Create an import (which you know would work without errors) and stop before the end, to let in "in progress" : you may stop at step 2, right after clicking "Suivant" in step 1
2. Go to see the report of this import, and observe that it is still in progress ("EN COURS" appearing on the top right), and that there are only three panels ; "Description de l'import", "Correspondances" and "Données invalides" ; but no panel "Données importées" visible (fixed by the present commit)
3. Finalize the import : go until through all steps, until step 5, and launch the check (click on "Lancer la vérification") while asserting that a celery worker is running locally / then, after checks are done, confirm import of observations (click on "Importer vos N observations valides".
4. Go to the report, and observe that the import is done ("TERMINE" on the top right) and that a 4th panel "Données importées" is now visible.