decimate sankey data (~1/10th the JS for large elections)

[artoonie]

On a 33-round election, before this change:

After:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.04%. Comparing base (59788d9) to head (d4bbce6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #614      +/-   ##
==========================================
+ Coverage   96.03%   96.04%   +0.01%     
==========================================
  Files         132      132              
  Lines        6306     6301       -5     
==========================================
- Hits         6056     6052       -4     
+ Misses        250      249       -1     

Files with missing lines	Coverage Δ
visualizer/sankey/graphToD3.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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