add environment variable CABOT_RESULTS_RETENTION_DAYS

[thomasleveil]

This pull requests allows to change the number of days for data retention by setting the CABOT_RESULTS_RETENTION_DAYS environment variable.

[codecov]

Codecov Report

Merging #669 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #669   +/-   ##
=======================================
  Coverage   80.91%   80.91%           
=======================================
  Files          46       46           
  Lines        2955     2955           
  Branches      179      179           
=======================================
  Hits         2391     2391           
  Misses        505      505           
  Partials       59       59           

Impacted Files	Coverage Δ
cabot/cabotapp/views.py	70.89% <ø> (ø)
cabot/celery.py	100.00% <ø> (ø)
cabot/settings.py	68.42% <ø> (ø)
cabot/cabotapp/tasks.py	61.66% <100.00%> (ø)

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[dbuxton]

Only one small request

[s7anley]

Hey @dbuxton 👋 Any chance this could get merged and released?