Validate policy files using schema

[leplatrem]

Currently we have some ifhere and there to validate the policies files content.

We could get rid of them and replace them with a JSON schema validation.

Having a JSON schema would also allow us to build a simple UI using react-jsonschema-form to author policies files online :)

[mostlygeek]

I wonder if we should build the policy validator as a separate CLI tool. I'd like to encourage people to source control their policy files. If we had a CLI tool we can use that in CI to validate, lint and sanity check their policies in an automated way.