Disallow usage of feature flags without a definition

This issue describes an approach to disallow usage of feature flags without YAML definition.

Proposal

1. Load feature flags definition in production
2. Disallow changing feature flag state via API if feature flag does not have YAML definition
3. Allow "to skip validation" with --force
4. Present in Features API the information if the feature flag has_definition
5. Extend ChatOps to support Valid and Obsolete feature flags and --force

Steps

1. Introduce a change in Feature Flags API to validate feature flag on POST
2. Return information on GET
3. Make a change to ChatOps to allow --force