Make Slack notifications configurable in the configurator

### Description

https://github.com/alleyinteractive/create-wordpress-project/blob/7f22e6a4a78ecc668c9dfb270e8c3962bec96b40/ci-templates/.github/workflows/copy-to-vip.yml#L28-L33

The CI Template files still rely on Slack webhooks, but internally at Alley our practice is to use a different method of surfacing deploy information in Slack. To make this more dynamic for both Alley and general-purpose use cases, we should prompt the user for the Slack webhook URL to use, and if left blank, the Slack webhook configuration should be removed from these files.

### Use Case

Removing the Slack deploy workflows when unused will reduce the amount of cruft present after running the initial setup.