Rename master git branch to main

[blimmer]

Many communities, both on GitHub and in the wider Git community, are renaming the default branch name of their repository from master -> main on the recommendation of the Software Freedom Conservancy. It would be nice to follow suit and rename the default branch of this project (and other Parabol projects as well).

GitHub now provides a UI-based way of updating the default branch, described here.

The biggest things that would need to be manually addressed:

1. All local copies of the repository need to be updated to point at main. There's a pretty easy copy/paste solution described in the GitHub docs. The steps also appear as a tooltip in the GitHub UI after the rename.
2. We'd need to update any configuration that pulls the code automatically. This code search didn't result in too many results, so it seems like it might not be too impactful. cc @adaniels-parabol - would this have significant impact on infra as code?

[adaniels-parabol]

@blimmer no major issues or concerns here on infra side. There may need to be an update to public-saas side with regard to the dokku config and on some of the server, but you referenced the solution in the issue above. I support this by the way +1 for bringing this up.