Send to debug log CDN purges from jobs

[david-a-wheeler]

Send a debug log when a job does a CDN purge. It's otherwise difficult to see if it happened.

To see the debug log results, you need to see it as described in implementation.md. E.g.:

heroku config:set --app staging-bestpractices RAILS_LOG_LEVEL=debug

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (a7ff9f4) to head (4c656f6).
⚠️ Report is 587 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2261   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files          55       55           
  Lines        2129     2130    +1     
=======================================
+ Hits         2128     2129    +1     
  Misses          1        1           

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