Rename master branch

The `master` term can be offensive, and so GitHub and others are encouraging switching to a more neutral term. `main` seems to be a good choice; some others would be `latest`, `trunk`, `stable` or `develop`. It sounds like GitHub will be publishing guidance on how to accomplish this soon. Otherwise, a quick search turned up these recent guides:

https://dev.to/rhymu8354/git-renaming-the-master-branch-137b

https://www.hanselman.com/blog/EasilyRenameYourGitDefaultBranchFromMasterToMain.aspx

I'm calling this a "documentation" issue because the pieces needed are probably more on the documentation side than anywhere else, and I don't think this will require any actual code changes.

----

Update (2021-03-08)

Here are the steps I can think of. I think we should do them in this order:

- [ ] Change documentation: rename master to main
- wiki (start by reverting commit `443e84d`, but there may be more to do than that)
- any README files that reference master
- any mentions of master in doc/source
- [ ] Rename branch on GitHub
- [ ] Send instructions to developers (see https://github.com/ESCOMP/CTSM/issues/1048#issuecomment-793203096)
- Relevant email list(s) (just ctsm-dev)
- Message the ctsm team on GitHub