Add muted class to author name in repo commit list (#29989)
Before:
![image](https://github.com/go-gitea/gitea/assets/18380374/f6b3728c-ed9a-4e47-8755-89373235dff2)
After:
![image](https://github.com/go-gitea/gitea/assets/18380374/272c85e3-620d-4758-ae4d-ad90b54e142c)
If repo is a mirror, external user's name will be white, but if user is
existed, then you will see blue names and white names together:
![image](https://github.com/go-gitea/gitea/assets/18380374/747622da-56e3-4162-b391-919787a8cee4)
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Co-authored-by: silverwind <me@silverwind.io>