Use multi-file upload for "images" field

Closes #62

apps/files/models.py

@@ -1,5 +1,6 @@
 from django.contrib.auth.models import User
 from django.db import models
+from django.utils.translation import ugettext as _
 
 from apps.map.models import CaseStudy, CaseStudyDraft
 
@@ -9,7 +10,7 @@ class BaseFile(models.Model):
         upload_to='.',
     )
     user = models.ForeignKey(
-        User, related_name='files'
+        User, related_name='%(class)s'
     )
 
     class Meta:
@@ -21,3 +22,21 @@ class BaseFile(models.Model):
 
 class File(BaseFile):
     pass
+
+
+class ImageFile(BaseFile):
+    caption = models.CharField(
+        verbose_name=_("Image caption"),
+        max_length=240,
+        default=None,
+        null=True,
+        blank=True,
+    )
+
+    credit = models.CharField(
+        verbose_name=_("Image credit"),
+        max_length=240,
+        default=None,
+        null=True,
+        blank=True,
+    )