Use multi-file upload for "images" field

Closes #62

apps/files/migrations/0004_auto_20180530_0308.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.6 on 2018-05-30 03:08
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('files', '0003_auto_20180526_1547'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='imagefile',
+            name='caption',
+            field=models.CharField(blank=True, default=None, max_length=240, null=True, verbose_name='Image caption'),
+        ),
+        migrations.AlterField(
+            model_name='imagefile',
+            name='credit',
+            field=models.CharField(blank=True, default=None, max_length=240, null=True, verbose_name='Image credit'),
+        ),
+    ]