Move respond_with to the responders gem

respond_with (and consequently the class-level respond_to)
are being removed from Rails. Instead of moving it to a 3rd
library, the functionality will be moved to responders gem
(at github.com/plataformatec/responders) which already provides
some responders extensions.
Remove usafe of respond_to in ActionView tests
Remove more references to respond_with
Merge pull request #16526 from rails/jv-no-responders

Move respond_with to the responders gem