Set a less erratic threshold for logging

There must now be no new logs for 100 data points. In testing on
production this seems to cause an alert after around 3 minutes.
Merge pull request #691 from alphagov/fix-logging-rate-check

Measure logging rate over 100 datapoints (around 3 minutes) to reduce false positives
Update stagecraft postactivate so it works
Merge pull request #693 from alphagov/update-stagecraft-postactivate

Update stagecraft postactivate so it works