Move YAML validation logic to within the CLI

[rohan-at-sentry]

There shouldn't be a need to make a network request to validate the YAML - we can likely do this within the CLI itself now, as it's no longer in bash. We don't really provide a way to communicate if a config is invalid, this is a step in that direction.

[nicholas-codecov]

If we're removing the ability to validate the YAML via an endpoint can you also make sure that we remove that copy from the UI. In the repo settings we have a snippet telling people how to validate their YAML.