Initial data clean-up for ancillary files

[rmbielby]

Pull request overview

I've added the ancillary files that aren't needed by the dashboard to .gitignore so that they don't bloat the repo.

Whilst I've been looking through the repo, I've also:

• Adjusted the deploy script to try and sidestep some known recent issues
• Changed the cookie handling to use the standard dfeshiny module

Pull request checklist

Please check if your PR fulfils the following:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Tests have been run locally and are passing (run_tests_locally())
• Code is styled according to tidyverse styling (checked locally with tidy_code())