Minor enhancements

* refactors eco account rest surface calculation into designated method get_available_rest()
* refactors placeholder setter for BaseForm
* adds automatic eco account identifier generating on saving
* fixes ServerMessageImportance template tag

intervention/forms.py

@@ -481,7 +481,7 @@ class NewWithdrawForm(BaseModalForm):
         self.is_intervention_initially = False
 
         # Add a placeholder for field 'surface' without having to define the whole widget above
-        self.fields["surface"].widget.attrs["placeholder"] = "0,00"
+        self.add_placeholder_for_field("surface", "0,00")
 
         # Check for Intervention or EcoAccount
         if isinstance(self.instance, Intervention):