Change a channel's branch binding, whether it follows that branch, or both.

The scope and role are create_channel's: channel:* and a MEMBER. The branch does not have to exist in the repo yet. channels/service.py::set_binding validates the binding and journals the change.

Turning follows_branch on also takes deploy:*, because from then on each GitHub push deploys the channel. It needs a GitHub connection and a binding (channels/service.py::set_follows_branch). Turning it off is always accepted. A body that carries both fields applies the binding first, so one request can bind a branch and follow it.