fix memory exhaustion on webapp creation

was introduced by 88a62c53

django/allgo/main/forms.py

@@ -193,7 +193,7 @@ class WebappForm(forms.ModelForm):
         # - value is converted into bytes
         return (self.cleaned_data["memory_limit_mb"]
                 if request.user.is_superuser
-                else config.env.ALLGO_WEBAPP_DEFAULT_MEMORY_LIMIT_MB
+                else int(config.env.ALLGO_WEBAPP_DEFAULT_MEMORY_LIMIT_MB)
                 ) * 1024**2
 
     class Meta: