Update modified_on whenever flow history changes

[rowanseymour]

Addresses https://github.com/nyaruka/mailroom/issues/615

[codecov]

Codecov Report

Merging #617 (962e373) into main (57d3991) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #617      +/-   ##
==========================================
+ Coverage   62.32%   62.42%   +0.09%     
==========================================
  Files         126      127       +1     
  Lines        8884     8894      +10     
==========================================
+ Hits         5537     5552      +15     
+ Misses       2482     2477       -5     
  Partials      865      865              

Impacted Files	Coverage Δ
core/handlers/noop.go	100.00% <ø> (ø)
core/handlers/flow_entered.go	100.00% <100.00%> (ø)
core/handlers/session_triggered.go	100.00% <100.00%> (ø)
core/handlers/sprint_ended.go	100.00% <100.00%> (ø)
core/models/events.go	70.52% <100.00%> (+0.31%)	⬆️
core/models/sessions.go	53.44% <100.00%> (-0.26%)	⬇️

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