merge list of config files

[tim-thacker-nullify]

Description

Refactor the MergeConfigFiles function to take in a list of config files. This is in preparation for providing an extended hierarchy of config files:

• system default
• dashboard settings
• .github-private
• repo

Checklist

• [ ] I have added one of the patch, minor, major or no-release labels
• [ ] I have linked an issue from this repository using the Development option
• [ ] I have performed a self-review of my own code
• [ ] I have checked for redundant or commented out code
• [ ] I have commented my code where I can't make it self-documenting
• [ ] I have made corresponding changes to the documentation
• [ ] I have added any appropriate tests