macportsbot: detect invalid maintainer GitHub handles

[chrstphrchvz (Christopher Chavez)]

Maintainers are not notified by macportsbot if they have changed their GitHub handle (or if they deleted their GitHub account, but in that case the maintainer would probably have to be emailed). I have noticed this multiple times, but I believe it is unlikely that others notice this situation (the only indication is that @handle in macportsbot’s comment isn’t shown in boldface), so it would help if macportsbot detects this and says something.

It is usually possible to retrieve the maintainer’s new handle manually, such as by navigating the blame feature on GitHub and/or looking for past PRs made by that user. It may be possible to automate this, for example if macportsbot caches user IDs for valid handles (e.g. 7941193 for @chrstphrchvz) and then looks up the new handle from the user ID if it becomes invalid.

(Yes, I understand that it is likely maintainers’ responsibility to ensure their contact info in portfiles is up-to-date.)