Fix deprecation warnings for Django 1.11 and Django 2.0.

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

mayan/apps/mailer/views.py

@@ -1,10 +1,10 @@
 from __future__ import absolute_import, unicode_literals
 
 from django.contrib.sites.models import Site
-from django.core.urlresolvers import reverse, reverse_lazy
 from django.http import Http404, HttpResponseRedirect
 from django.shortcuts import get_object_or_404
 from django.template import Context, Template
+from django.urls import reverse, reverse_lazy
 from django.utils.html import strip_tags
 from django.utils.translation import ungettext, ugettext_lazy as _