editor form: do not require crop_to_level

src/c3nav/mapdata/migrations/0012_auto_20161204_1544.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.4 on 2016-12-04 15:44
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mapdata', '0011_auto_20161204_1041'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='obstacle',
+            name='crop_to_level',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='crops_obstacles', to='mapdata.Level', verbose_name='crop to other level'),
+        ),
+    ]

src/c3nav/mapdata/models/geometry.py

@@ -184,7 +184,7 @@ class Obstacle(GeometryMapItemWithLevel):
     """
     An obstacle
     """
-    crop_to_level = models.ForeignKey('mapdata.Level', on_delete=models.CASCADE, null=True,
+    crop_to_level = models.ForeignKey('mapdata.Level', on_delete=models.CASCADE, null=True, blank=True,
                                       verbose_name=_('crop to other level'), related_name='crops_obstacles')
 
     geomtype = 'polygon'