Idea to stop PR spam

Hi, Followup on https://github.com/academicpages/academicpages.github.io/pull/3484#issuecomment-3835104074 My idea: if any PR is opened with a title that does not start with `UPSTREAM: `it should be closed immediately with a helpful message to the user. In that case users who push here by mistake would get their PR closed before even pushing a second commit, greatly reducing the number of spam emails. Likewise, it would make it easy to add rule to our mail clients to auto delete those mails because of the subject line.