Release v1.4.20 - add pagination to pager command

[echoboomer]

Describe your changes

• Adds pagination logic for collections to handle internal pagination.
• Adds pagination logic to the pager command to iterate via a globally controllable per_page value. This sends a separate message for every per_page items in a collection to avoid breaking Slack's block limit.
• Formatting changes in a few places.
• Refactors method used to collect on call data from PagerDuty.

Issue number and link

#328

Checklist before requesting a review

• I have properly bumped all version refs by running ./scripts/version-bump.sh old new from project root - for example ./scripts/version-bump.sh 1.4.8 1.4.9
• I have added or changed tests as part of this pull request
• I will open a pull request for the Helm chart to bump the app and chart versions