If a flow start task creates new contacts, save those back to the start

[rowanseymour]

Addresses #361

[codecov]

Codecov Report

Merging #362 into master will increase coverage by 0.11%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
+ Coverage   54.01%   54.13%   +0.11%     
==========================================
  Files         112      112              
  Lines        8010     8025      +15     
==========================================
+ Hits         4327     4344      +17     
+ Misses       2959     2955       -4     
- Partials      724      726       +2     

Impacted Files	Coverage Δ
tasks/starts/worker.go	55.43% <60.00%> (-0.25%)	⬇️
models/starts.go	46.51% <100.00%> (+8.37%)	⬆️

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