Fragment now works on both React and React Native, so use it.

Contributing all buttons in one place goes against the designs that we
set out at the beginning of the project's rewrite and that multiple of
us have been following since then.

This abstraction represents an action which can go anywhere in a toolbox (be
that the main toolbar or the overflow menu) and it's platform independent.

It does not depend on Redux, thus making it stateless, which facilitates its use
in stateful button implementations as well as stateless ones.