Pin cookiecutter versions in generated backend Makefile

[davisagli]

The backend Makefile generated by cookiecutter-plone-starter should pin the versions of cookiecutter and cookiecutter-zope-instance that it uses, so that the commands continue to do the same thing even if new versions of cookiecutter or cookiecutter-zope-instance are released in the future.

[jensens]

If we switch in the backend to use mxmake: there all this is prepared in a very flexible way. This would also reduce the burden to manage the same on two places.

I volunteer to implement this if there is a go. So, this probably needs a new issue.