Email threading for github PR notifications

[preames]

Unless I've missed something digging through the mailer script (https://github.com/llvm/llvm-admin/blob/main/AWS/Lambda/PREmailer/pr_emailer.py), and the resulting email headers, it doesn't look like we're setting appropriate headers to assist email clients in threading PR responses. Or at least, I use Thunderbird, and can't figure out how to get the notices threaded.

I don't have much experience with this part of email sending, but I understand there's a In-Reply-To header which should be set. The challenge is going to be that this requires state in the mailer as we'll have to track the message idea for each previous message in the thread.

If there's an alternate way to solve this - i.e. I just haven't found the right client option yet - feel free to suggest alternative approaches.

[pogo59]

It might be a client thing. Gmail is threading the emails appropriately, afaict.

[llvmbot]

@llvm/issue-subscribers-infrastructure

[tstellar]

Here is my attempt at implementing this: llvm/llvm-admin#2