Deprecate nbdev-template and bring its files & functionality into nbdev

This is a better solution to the underlying problem described in https://github.com/fastai/nbdev/issues/1091, which is that with the current setup any breaking changes in nbdev-template will affect _all_ versions of nbdev.

There's no need to implement this just yet. A good time might be when we next want to make a breaking change to the template.