Stopping workflows

It is possible to stop workflows from gush-control now.
This PR depends on [pausable workflows PR in gush](https://github.com/lonelyplanet/gush/pull/2).
Please merge [infinite scrolling PR before this one](https://github.com/pokonski/gush-control/pull/5). They don't really depend on each other, but it'd be more consistent this way.