feat: generate GitLab CI/CD config using a subtemplate

[erral]

As an alternative to #239 in this PR we add the GitLab CI/CD config as a subtemplate

[stevepiercy]

The ci_gitlab/README.md needs some love. I think @ericof had an idea of how to generate all READMEs from boilerplate, but I'm not sure where that is at.

[stevepiercy]

Much empty, so sad. 😭

[pbauer]

Where is the check-target in the Makefile? Do you mean make lint?

[erral]

yes, I meant make lint. I had an older version of cookieplone-templates in place where make check was the command, sorry about that :(

[pbauer]

You only change the - to a / in the Makefile. The Makefiles in project_settings and classic_project_settings are not identical, so this is not ok.
It might be best to have one Makefile for both (as a subtemplate?) and make some targets conditional and the separator also conditional.

[erral]

I have reworked this, reviewed and added tests.

The only thing that I don't like is overwriting the Makefile to modify the docker image name.

@pbauer suggested a way to address it creating a subtemplate for Makefiles, but I haven't checked it yet.

[acsr]

I would be great if we can push this or the other #239 through the door.

I am not sure which one is worth testing currently. I would be nice if @ericof or @erral can give a flag if this is still worth a try.

This progress would also fix #176