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/main/views/test_template_history.py

@@ -1,4 +1,3 @@
-import json
 from flask import url_for
 
 
@@ -54,7 +53,6 @@ def test_view_template_versions(
 ):
     service_id = fake_uuid
     template_id = fake_uuid
-    version = 1
     resp = logged_in_client.get(url_for(
         '.view_template_versions',
         service_id=service_id,