Fix deprecation warnings for Django 1.11 and Django 2.0.

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

mayan/apps/document_signatures/models.py

@@ -3,8 +3,8 @@ from __future__ import unicode_literals
 import logging
 import uuid
 
-from django.core.urlresolvers import reverse
 from django.db import models
+from django.urls import reverse
 from django.utils.encoding import force_text, python_2_unicode_compatible
 from django.utils.translation import ugettext_lazy as _
 
@@ -37,8 +37,8 @@ class SignatureBaseModel(models.Model):
     """
 
     document_version = models.ForeignKey(
-        DocumentVersion, editable=False, related_name='signatures',
-        verbose_name=_('Document version')
+        DocumentVersion, editable=False, on_delete=models.CASCADE,
+        related_name='signatures', verbose_name=_('Document version')
     )
     # Basic fields
     date = models.DateField(

mayan/apps/document_signatures/tests/test_links.py

@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 
 from django.core.files import File
-from django.core.urlresolvers import reverse
+from django.urls import reverse
 
 from documents.tests.literals import TEST_DOCUMENT_PATH
 from documents.tests.test_views import GenericDocumentViewTestCase

mayan/apps/document_signatures/views.py

@@ -4,9 +4,9 @@ import logging
 
 from django.contrib import messages
 from django.core.files import File
-from django.core.urlresolvers import reverse
 from django.http import HttpResponseRedirect
 from django.shortcuts import get_object_or_404
+from django.urls import reverse
 from django.utils.encoding import force_text
 from django.utils.translation import ugettext_lazy as _