Release v1.9.0 - Reorganize postmortems, other quality of life fixes.

[echoboomer]

Describe your changes

• Renames all use of "rca" to "postmortem" because that's the standard and it's confusing to do both.
• API will now always run, but only the health check route is served when the API is "disabled" so health checks still work.
• Link to postmortem now gets added to the digest message if one is created.

Note: This requires a database migration and updates to the configuration. Please see all notes.

Issue number and link

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

DATABASE MIGRATION REQUIRED

You can run make run_migrations from the root of this project, OR you can simply drop the incidents table and the application will recreate it using the correct name. You can also optionally just rename the rca column to postmortem.

IF YOU ARE USING THE CONFLUENCE INTEGRATION

You must rename auto_create_rca to auto_create_postmortem in config.yaml.