Add git workflows

[ehartley]

I added github and gitlab workflow scripts that will run the make validate-mappings command and a new field called github_or_gitlab to the cookiecutter.json file for the user to select which option they want. The post_gen_project.py script will remove the unneeded files (e.g., the .gitlab-ci.yml file will be removed if 'github' was selected).

Other changes:

• Added a pre_gen_project.py script that terminates the project generation if the provided project_name contains any special characters other than _ or -.
• In cookiecutter.json, removed "yo" and reordered the fields to a more logical sequence (in my opinion).
• Added .gitignore
• Reduced the contents of registry.yml so that it only contains a small sample of entries.

Note: I haven't tested these changes. I will do so after they are merged.

[matentzn]

Thanks for your review @hrshdhgd and for the PR @ehartley - you can take it from here!

[ehartley]

Thank you @hrshdhgd and @matentzn!