Add initial Django 2.0 support.

database_files/settings.py

 import os
 
 from django.conf import settings
-from django.core.urlresolvers import reverse
+from django.urls import reverse
 
 # If true, when file objects are created, they will be automatically copied
 # to the local file system for faster serving.

database_files/storage.py

@@ -7,7 +7,7 @@ from six import StringIO
 from django.conf import settings
 from django.core import files
 from django.core.files.storage import FileSystemStorage
-from django.core.urlresolvers import reverse
+from django.urls import reverse
 
 from database_files import models
 from database_files import utils

database_files/urls.py

 
-try:
-    # Removed in Django 1.6
-    from django.conf.urls.defaults import patterns, url
-except ImportError:
-    from django.conf.urls import patterns, url
+from django.urls import re_path
 
-urlpatterns = patterns('',
+from database_files import views
+
+urlpatterns = [
 #    url(r'^files/(?P<name>.+)$',
 #        'database_files.views.serve',
 #        name='database_file'),
-    url(r'^files/(?P<name>.+)$',
-        'database_files.views.serve_mixed',
+    re_path(r'^files/(?P<name>.+)$',
+        views.serve_mixed,
         name='database_file'),
-)
+]