Fix CompensationAction unit None

* adds correct declaration of unit (qm -> m2) for template rendering
* adds migration to transform existing qm units to m2

compensation/models/action.py

@@ -19,8 +19,9 @@ class UnitChoices(models.TextChoices):
     """
     cm = "cm", _("cm")
     m = "m", _("m")
+    m2 = "m2", _("m²")
+    m3 = "m3", _("m³")
     km = "km", _("km")
-    qm = "qm", _("m²")
     ha = "ha", _("ha")
     st = "pcs", _("Pieces")  # pieces