json_response_helpers.rb

This is not a good idea to memoize `json_response` using an instance
variable because `rspec-retry` doesn't clear instance variables on
retries, only `let` variables.

This will avoid issues where retries would fail on a different line that
the original failure, blurrying what's the real failure.

Also, automatically add api: true to specs under
/spec/requests/(ci/)?api/, and include JsonHelpers in controller, request
and API specs.

Signed-off-by: Rémy Coutable <remy@rymai.me>