Combine Slack Integration into this and re-brand Sonatype Webhook Handler

[madpah]

Plan is for a single executable application to handle webhooks from Sonatype Platform.

• [x] Port in Slack webhook handler from https://github.com/sonatype-nexus-community/sonatype-slack-apps-integration
• [ ] Deprecate https://github.com/sonatype-nexus-community/sonatype-slack-apps-integration
• [x] Update documentation