Fix Unicorn restarts during deployments

- Ensure that program code gets reloaded by completely stopping
  and, later, restarting.
- Augment and reorder deployment tasks, and remove handlers.

Restarts are done as `api' and `frontend' role deployment tasks,
rather than as pre_tasks and post_tasks, for consistency between
both roles.

I've run deployments on development and staging and they've succeeded.