Add slug check

[froggleston]

Currently when building a workshop website, there is no check to validate a repository name as a slug. This PR adds a check that will fail if the repo name does not match YYYY-MM-DD-[site][-format] if the carpentry specified in the _config.yaml is one of our official lesson programs. This will mean the workshop website will fail to build completely, and the user is guided to rename their repo in the Settings menu.

When using cp or incubator as the carpentry option, the check will only warn about the slug as this could be an incubator lesson-based workshop.

[froggleston]

For added context, carpentries/styles#551 should no longer apply - this template should only now be used for workshop websites and not lessons.

[tobyhodges]

@froggleston thanks for contributing this. The equivalent check should also be added to https://github.com/carpentries/training-template/, I think?

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[froggleston]

cp carpentries workshops can be used for incubator workshops, which may or may not have a valid slug. So, when using cp give a warning if the slug isn't valid, but do not fail.

[froggleston]

@froggleston thanks for contributing this. The equivalent check should also be added to https://github.com/carpentries/training-template/, I think?

Yup, I'll work on this tomorrow!

[tobyhodges]

Looks good to me, thanks @froggleston 🙌