Add a primary author

[rkotze]

Summary

A new user to Git and Git Mob might not have a primary author set globally or locally. Help the user set a primary author with a name and email using the Git Mob UI.

Currently, the extension breaks, shows no UI of co-authors making it difficult to know what to do next.

Motivation

Make it easier and reduce the frustration for the user to get started.

Describe alternatives you've considered

Currently, there is a link and instructions in the readme to set up correctly