Revert #1421

[apiology]

This reverts #1421 until we can validate the change further.

The PR changed our prevalence ratio formula from:
1000/(day_i + 50) * (positivity-rate)^0.5 + 1
to
1500/(day_i + 50) * (positivity-rate)^0.5 + 1

However, I do not believe that reflects the actual March 2, 2021 update on the referenced paper. In fact, I believe it rolls us back to a formula used prior to the December 10th, 2020 update:

December 10, 2020 Update: We adjusted the constant in the prevalence ratio formula from a = 1500 / (day_i + 50) to a = 1000 / (day_i + 10). This results in a slightly higher prevalence ratio in the beginning of the pandemic and a lower prevalence ratio currently.

I believe that the author of the PR assumed that the update notices at the top of the paper indicated that the contents had been updated, noticed that the formula inside didn't match our formula, and changed our formula to match. I believe that the contents of the paper had not actually changed for many of the listed updates, and the updates instead referred to the actual live model website the paper described.

Here’s the page prior to the March 2, 2021 update:

https://web.archive.org/web/20210227192800/https://covid19-projections.com/estimating-true-infections-revisited/

and here it is today:

https://covid19-projections.com/estimating-true-infections-revisited/

Notice that the paper uses the 1500/(day_i + 50) * (positivity-rate)^0.5 + 1 formula in both.

[codecov]

Codecov Report

Merging #1693 (6eefbe6) into main (e5496e6) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1693      +/-   ##
==========================================
- Coverage   56.70%   56.68%   -0.02%     
==========================================
  Files         103      103              
  Lines        2580     2579       -1     
  Branches      536      536              
==========================================
- Hits         1463     1462       -1     
  Misses        967      967              
  Partials      150      150              

Impacted Files	Coverage Δ
src/pages/Calculator.tsx	79.80% <ø> (ø)
src/posts/paper/7-basic-method.ts	0.00% <ø> (ø)
src/posts/paper/99-changelog.ts	100.00% <ø> (ø)
src/data/calculate.ts	86.18% <100.00%> (ø)
test_update_prevalence.py	100.00% <100.00%> (ø)
update_prevalence.py	50.04% <100.00%> (-0.05%)	⬇️

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