Improvement: {bslib} (-ify) the UI

[JT-39]

Improvement Overview

{bslib} is the new gold standard for shiny dashboard applications. The package provides a modern UI toolkit for Shiny and R Markdown based on Bootstrap. Previous UI packages such as {shinydashboard} are being depreciated and so should be moved away from.

The DfE shiny-template currently uses some of the older UI functions such as shiny::fluidPage() and shinyWidgets::useShinydashboard(). This needs to be refactored to use the equivalent {bslib} functions where possible. There are other major benefits to doing this other than moving away from depreciated code, such as improvements in accessibility (due to the fillable feature in the UI functions) and improved customisation through easier css and access to the Bootstrap and Bootswatch functionalities.

Current Issue

The DfE shiny-template uses legacy shiny UI code, such as shiny::fluidPage() and shinyWidgets::useShinydashboard().

Proposed Enhancement

To replace and upgrade this code with the equivalent (or best) {bslib} functions, such as bslib::page_fillable().

Impact

This must result in the same functionality. There should be benefit from improved accessibility (due to the fillable feature of {bslib} UI functions) and a smoother/ more aesthetic overall UI experience.

Additional Information

This will likely be an iterative process and should also try to be extracted so that it can be contributed to the DfE shiny-template

[JT-39]

Re-opening for the reason's given in the Additional Information of this issue and the Bslib ify shiny template pull request