improve rendering speed by caching LevelGeometry in database

src/c3nav/mapdata/migrations/0037_level_geoms_cache.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.6 on 2017-10-19 11:21
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mapdata', '0036_geometry_bounds'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='level',
+            name='geoms_cache',
+            field=models.BinaryField(default=b''),
+            preserve_default=False,
+        ),
+    ]