Changes to GET /district/:district_key/events/keys

api tag list removed

added the non-success response with the status 404

added the media type application/json for the response with the status 401

the optional response header Last-Modified removed for the status 200

deleted the header request parameter If-Modified-Since

added the new optional header request parameter If-None-Match

added the media type application/json for the response with the status 200