Bug 1372640 - Add django-filters to INSTALLED_APPS

The jobdetail browsable API pages were returning HTTP 500 since as of
carltongibson/django-filter#580 it's necessary to add django-filter to
INSTALLED_APPS: https://django-filter.readthedocs.io/en/latest/guide/install.html

treeherder/config/settings.py

@@ -142,6 +142,7 @@ def server_supports_tls(url):
     'rest_framework_swagger',
     'hawkrest',
     'corsheaders',
+    'django_filters',
     'graphene_django',
     # treeherder apps
     'treeherder.model',