Modernize terminology #3

We should modernize our terminology to follow the times.

Use blocklist/allowlist instead of blacklist/whitelist, following a Linux mailing list
Use main instead of master for the central branch, following GitLab/GitHub
- We expect this one to cause some breakage with Maat Builder -- manual intervention is likely to be required, but can be as simple as removing all source directories.
- We can effect this with git branch -m master main && git push -u origin main.
- Gitea will need some cleanup afterwards by going to /settings/branches and setting the default branch to main before git push --delete origin master.

This issue will be updated if any other terms are found/proposed. Close criteria are completed remediation of complaints and no new complaints for 90 days.

We should watch for Gitea to implement some feature to set the default branch name like GitLab.

master->main conversions completed, for the most part.

Update: confirmed that Gitea allows setting the default branch name now.