Enable `make template` to be run without upgrading to the latest version of the template

There are two reasons to run `make template`:

1. There have been changes to the cookiecutter and you want to apply them to the current project
2. You've changed something in the current project's `cookiecutter.json` file (for example: the Python versions) and you want to apply that change

Currently if a project isn't up to date with the latest cookiecutter and you've changed something in `cookiecutter.json` and want to apply it you can't do that without also applying the changes from the cookiecutter: `make template` will both apply the changes you've made in `cookiecutter.json` (e.g. changing the Python versions) and apply the changes from the cookiecutter at once.

It would be nice if the project's current version of the cookiecutter (commit SHA) were saved in `cookiecutter.json` and `make template` used that version of the cookiecutter instead of the latest. Then you could make changes to `cookiecutter.json` and run `make template` to apply them without also upgrading the project to the latest version of the cookiecutter. `make template args=upgrade` could be used to upgrade to the latest cookiecutter.

`make template` already has some of what we'd need to implement this. You can optionally put a `"checkout"` setting in `cookiecutter.json` and [`make template` will pass it](https://github.com/hypothesis/cookiecutters/blob/78488ea1c484307d2b55a8c2383d3e50e1573f81/_shared/project/bin/make_template#L26) to [cookiecutter's `checkout` option](https://cookiecutter.readthedocs.io/en/stable/cli_options.html?highlight=checkout#cmdoption-cookiecutter-c). Currently `"checkout"` in `cookiecutter.json` is `null` by default. We'd need to make it put in the SHA of the cookiecutter commit that was used instead.