Fix deprecation warnings for Django 1.11 and Django 2.0.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_comments/models.py

@@ -19,12 +19,12 @@ logger = logging.getLogger(__name__)
 @python_2_unicode_compatible
 class Comment(models.Model):
     document = models.ForeignKey(
-        Document, db_index=True, related_name='comments',
-        verbose_name=_('Document')
+        Document, db_index=True, on_delete=models.CASCADE,
+        related_name='comments', verbose_name=_('Document')
     )
     user = models.ForeignKey(
-        settings.AUTH_USER_MODEL, editable=False, related_name='comments',
-        verbose_name=_('User'),
+        settings.AUTH_USER_MODEL, editable=False, on_delete=models.CASCADE,
+        related_name='comments', verbose_name=_('User'),
     )
     # Translators: Comment here is a noun and refers to the actual text stored
     comment = models.TextField(verbose_name=_('Comment'))

mayan/apps/document_comments/tests/test_api.py

@@ -1,8 +1,8 @@
 from __future__ import unicode_literals
 
 from django.contrib.auth import get_user_model
-from django.core.urlresolvers import reverse
 from django.test import override_settings
+from django.urls import reverse
 
 from rest_framework.test import APITestCase
 

mayan/apps/document_comments/views.py

@@ -1,7 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
-from django.core.urlresolvers import reverse
 from django.shortcuts import get_object_or_404
+from django.urls import reverse
 from django.utils.translation import ugettext_lazy as _
 
 from acls.models import AccessControlList