Use multi-file upload for "images" field

Closes #62

apps/map/migrations/0065_casestudy_images.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.6 on 2018-05-26 15:47
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('files', '0003_auto_20180526_1547'),
+        ('map', '0064_auto_20180526_1536'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='casestudy',
+            name='images',
+            field=models.ManyToManyField(blank=True, related_name='image_for', to='files.ImageFile', verbose_name='Images'),
+        ),
+    ]