Re-factor, add in two-way sankey

[pwinskill]

A substantial PR, implemented so that we can include the full pathway Sankey diagram.
This has led to:

1. Refactoring of the sakey preparatin code, with more intuitive node and flow creation and labelling
2. Updating app to include two-way plot
3. Re-factor of app code to be simplier, modular and more robust

[codecov]

Codecov Report

❌ Patch coverage is 92.51969% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.75%. Comparing base (b150ccc) to head (18cfcb7).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
R/app_utils.R	81.69%	13 Missing ⚠️
R/gg_sankey_utils.R	95.16%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   46.89%   50.75%   +3.86%     
==========================================
  Files           9       10       +1     
  Lines         676      729      +53     
==========================================
+ Hits          317      370      +53     
  Misses        359      359              

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