Shiny template module

[JT-39]

Pull request overview

Started out with aim to modularise the cookie logic in shiny-template. Turns out this has already been done and is available in the {dfeshiny} package. I have now included this into the ui.R and server.R scripts and cleared out the old code.

I did make some minor improvements - namely creating a helper function to simply add the .css file to the ui.R script.

Pull request checklist

Please check if your PR fulfills 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 (shinytest2::test_app())
• Code is styled according to tidyverse styling (checked locally with styler::style_dir() and lintr::lint_dir())

What is the current behaviour?

Cookie logic is verbose in the server.R and ui.R.

What is the new behaviour?

This has been replaced with the dfeshiny::cookies_ functions in the server.R and ui.R. I also added a function to simply add the .css file into the ui.R, this is called set_css_style_sheet().

Anything else

NOTE: New updates are coming to {dfeshiny} which may break code (due to function name changes). Also a test should be written for the new set_css_style_sheet() function. (Plus check if this is okay practice.)