Replace Foreman with runit

Several months ago, there was some conversation if it was possible to make gdk run restart a process whenever it was stopped. This would be useful for example to update a service like gitaly.

There was an attempt to use nodemon instead of foreman, because that seemed to have the option to restart stopped processes.

But then I did some research. And you make gdk run start runsv and make that process start/restart gitaly:

• install runit -> apt install runit
• create gdk/gitaly/run with

exec bin/gitaly config.toml

But in fact, Jacob questioned if we wanted to replace Foreman completely. I didn't feel that was needed/logical at the time.

Other benefits

At #425 (closed) I realized there would be an added benefit to using runit. It would be easier to find the processes, because (I think) runit writes .pid files.