make sure static subdir 404s correctly

before each request, we put the current service on the flask session,
except for with the static folder, cos it's not needed.... except, if
we 404, then we return the 404 template, which checks if you're logged
in or not to display different nav bar items. This was crashing when
current_service wasn't set, so we now set it.

also cleaned up some imports and stuff in test files

tests/app/test_utils.py

@@ -3,7 +3,6 @@ from io import StringIO
 from csv import DictReader
 
 import pytest
-from freezegun import freeze_time
 
 from app.utils import (
     email_safe,
@@ -13,7 +12,7 @@ from app.utils import (
     Spreadsheet
 )
 
-from tests import notification_json, single_notification_json, template_json
+from tests import notification_json, single_notification_json
 
 
 def _get_notifications_csv(