Add a job class for generating excel for responses.
Add ResponsesController#generate_excel.

- Replaced the axlsx_rails gem with axlsx.
- Moved the `server_url` method to ApplicationController.
- Serialize response_ids for the job, instead of whole responses.
- Moved `filename_for_excel` to Survey model
- Removed the old `index.xlsx.axlsx`
Change the format of Survey#filename_for_excel to be urlsafe.