Track review activity

[pitaj]

It's common for the last activity on a PR to be irrelevant:

• comment from triage asking for status
• author asking for status
• rustbot informing about a merge conflict
• author fixing a merge conflict

Ideally, we would track review activity:

• exclude comments by the author
• exclude rustbot comments
• exclude comments by others with certain keywords like "triage", "status", "bump", etc
• include all activity by the assigned reviewer
• include code-review activity
• exclude drafts

This obviously isn't possible with the basic Github filters available, but I think it would give us a much better idea of how long PRs are sitting without activity. Merge conflict notifications alone likely result in a de-facto upper limit to time-since-updated.

[pitaj]

I think a simple version of this can be achieved by tracking how long individual PRs have been in the current state (by tracking the state of each PR individually). This would allow us to see how long PRs are being left "waiting-on-review" or "waiting-on-ACP", etc.