Allow to publicly access `enable` and `disable` endpoints in FF API

Summary

Currently, the following endpoints are allowed to access by us (for dogfooding purpose) and disallowed to access by customers/users, as it could break the feature flag UI due to the current UI incapability. If customers request to access it, we should ensure that it's not harmful on the UI part and safely rollout the endpoint.

POST   /api/:version/projects/:id/feature_flags/:name/enable(.:format)                   - Enable a strategy of a feature flag for an environment
POST   /api/:version/projects/:id/feature_flags/:name/disable(.:format)                  - Disable a strategy of a f

These endpoints are powerful as it supports the following capability as well:

Important note

We'll not remove feature_flag_api feature flag until we've resolved all issues described above

Originally discussed in !18368 (comment 229565793)

Edited by Shinya Maeda