Rename master branch

[prushforth]

It's been brought to my attention that the use of the term 'master' as the default branch name in git repositories is inherited racist terminology derived from 'master/slave', that can be offensive without intention.

For that reason, and out of a desire to not erect barriers to potential participants for no good reason, I suggest that we rename the master branch of this and all other repositories in the CG.

Perhaps 'trunk' or 'main' would be a good default. I will look into setting up a bare repo with a default branch of that name, which we might use as a template for future repos.

[NickFitz]

There's a useful article by Scott Hansleman on how to rename local and remote branches without breaking existing stuff: https://www.hanselman.com/blog/EasilyRenameYourGitDefaultBranchFromMasterToMain.aspx

I assume the procedure for synchronising the change with a local fork would be along the lines of the "Updating local clones" section of that article, but applied to "upstream" as well as "origin", but I'm not 100% sure about that. I'll give it a try with some of my own repositories from different accounts first, and report back.

[AmeliaBR]

One thing to consider is that Github Pages currently only supports using either a master or a gh-pages branch. For this repo, we already have a separate gh-pages branch that contains a static build of the document, so that's not an issue. But other repos are just static pages, so the main branch would need to be named gh-pages. (Or we can wait and see if GitHub responds to pressure and removes that restriction.)

Also, when making the change we would need to update the build actions from Travis CI to be based on the new branch name.