Refactor and implement download code natively

- Use modified port of Django 2.2 FileResponse.
- Remove Django DownloadView library.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/common/tests/base.py

@@ -2,8 +2,6 @@ from __future__ import absolute_import, unicode_literals
 
 from django.test import TestCase
 
-from django_downloadview import assert_download_response
-
 from mayan.apps.acls.tests.mixins import ACLTestCaseMixin
 from mayan.apps.converter.tests.mixins import LayerTestCaseMixin
 from mayan.apps.permissions.tests.mixins import PermissionTestCaseMixin
@@ -14,7 +12,7 @@ from mayan.apps.user_management.tests.mixins import UserTestMixin
 
 from .mixins import (
     ClientMethodsTestCaseMixin, ConnectionsCheckTestCaseMixin,
-    ContentTypeCheckTestCaseMixin, ModelTestCaseMixin,
+    ContentTypeCheckTestCaseMixin, DownloadTestCaseMixin, ModelTestCaseMixin,
     OpenFileCheckTestCaseMixin, RandomPrimaryKeyModelMonkeyPatchMixin,
     SilenceLoggerTestCaseMixin, TempfileCheckTestCasekMixin,
     TestViewTestCaseMixin
@@ -22,7 +20,8 @@ from .mixins import (
 
 
 class BaseTestCase(
-    LayerTestCaseMixin, SilenceLoggerTestCaseMixin, ConnectionsCheckTestCaseMixin,
+    LayerTestCaseMixin, SilenceLoggerTestCaseMixin,
+    ConnectionsCheckTestCaseMixin, DownloadTestCaseMixin,
     RandomPrimaryKeyModelMonkeyPatchMixin, ACLTestCaseMixin,
     ModelTestCaseMixin, OpenFileCheckTestCaseMixin, PermissionTestCaseMixin,
     SmartSettingsTestCaseMixin, TempfileCheckTestCasekMixin, UserTestMixin,
@@ -31,7 +30,6 @@ class BaseTestCase(
     """
     This is the most basic test case class any test in the project should use.
     """
-    assert_download_response = assert_download_response
 
 
 class GenericViewTestCase(