CHI-3571: Zero transcript retention support

[stephenhand]

Description

• Migrates existing service level conversation event handlers to the twilio lambda
• Adds support for registering handlers similar to the task router event handler.
• Adds a configuration flag to specify when helplines have a zero transcript retention policy
• Adds a conversation handler to delete conversations when they are closed when the helpline has a zero retention policy.
• Adds a programmable chat channel handler to delete chat channels when they are marked inactive when the helpline has a zero retention policy

Checklist

• Corresponding issue has been opened
• New tests added
• Feature flags added
• Strings are localized
• Tested for chat contacts
• Tested for call contacts

Other Related Issues

None

Verification steps

AFTER YOU MERGE

1. Cut a release tag using the Github workflow. Wait for it to complete and notify in the #aselo-deploys Slack channel.
2. Comment on the ticket with the release tag version AND any additional instructions required to configure an environment to test the changes.
3. Only then move the ticket into the QA column in JIRA

You are responsible for ensuring the above steps are completed. If you move a ticket into QA without advising what version to test, the QA team will assume the latest tag has the changes. If it does not, the following confusion is on you! :-P