geiq: switch to OneToOneField

To avoid warning:

    geiq.ImplementationAssessment.activity_report_file: (fields.W342)
    Setting unique=True on a ForeignKey has the same effect as using a
    OneToOneField.
        HINT: ForeignKey(unique=True) is usually better served by a
        OneToOneField.

itou/geiq/migrations/0001_initial.py

@@ -144,12 +144,11 @@ class Migration(migrations.Migration):
                 ("review_comment", models.TextField(blank=True, verbose_name="commentaire")),
                 (
                     "activity_report_file",
-                    models.ForeignKey(
+                    models.OneToOneField(
                         blank=True,
                         null=True,
                         on_delete=django.db.models.deletion.PROTECT,
                         to="files.file",
-                        unique=True,
                         verbose_name="document de synthèse",
                     ),
                 ),

itou/geiq/models.py

@@ -43,12 +43,11 @@ class ImplementationAssessment(models.Model):
 
     other_data = models.JSONField(verbose_name="autres données")
 
-    activity_report_file = models.ForeignKey(
+    activity_report_file = models.OneToOneField(
         File,
         verbose_name="document de synthèse",
         null=True,
         blank=True,
-        unique=True,
         on_delete=models.PROTECT,
     )