VariantsController has been merged to RepresentationsController, this
PR fixes outdated references to VariantsController in ActiveStorage documentation.
Since ActiveStorage::Blob::Representable unifies the idea of previews and
variants under one roof as representation, we may as well have the
controllers follow suit.
Thus ActiveStorage::RepresenationsController enters the fray. I've copied
the old tests for both previews and variants and unified those as well.