Modularise code

[matt-dray]

The proof of concept (#1) was not modularised. It should be done sooner rather than later.

At first, consider modules 'one-level' deep:

• provider dropdown
• strategy dropdown
• rates plot

Bearing in mind that the dropdown modules could later become submodules in an overarching 'navigation' module in future, or something like that.

Plot logic should go in a fct_plot.R along with other plots yet to be added. Similarly, table outputs handled in fct_tables.R. Any code in fct_*.R is prime for transplanting into a package, as per https://github.com/The-Strategy-Unit/nhp_planning/issues/417 and https://github.com/The-Strategy-Unit/nhp_planning/issues/418.

[matt-dray]

For now, I've left {bslib} UI structuring in app_ui.R so that modules handle the logic of the thing, not the UI of the thing.