Better UX for external contributors being delegated to

This is likely a very low need, but jotting it down as I was thinking about this issue: for reasons of more reliable attribution and tracking, the reviewer of a PR needs their email correctly set[^1]. This means if a community member is delegated and tries to approve a PR the bot will reject the review (or something).

Currently this just results in a feedback message telling the user to contact an admin but that's not really helpful as it provides no clue as to who the admins are, how to contact them, and what to ask for.

In reality it would make sense for the 'bot to state that the administrators have been messaged, and to message_post on the PR targeting either the runbot administrators or the contact / partner managers so they can check if configuring the external user for this makes sense.

[^1]: the author doesn't because in most cases the commits are correctly tagged, the two cases where it's not are merges (rebase-merge and merge) and in that case the bot is attributed the authorship of the merge commit[^2]

[^2]: with the specific exception of the head of a PR being a merge commit, in which case we essentially rebase that merge commit, in theory this is useful when the merge commit needs to be non-trivial (e.g. conflict resolution) though AFAIK it's very much a secret and essentially never used, it's more common to just rebase the entire thing and resolve all conflicts

Start by tracing the existing feedback message for delegated external contributors and the PR review handling described in the issue. Confirm how administrators or contact/partner managers are identified and how message_post is used; done means the bot tells the contributor that administrators were messaged and posts the relevant PR notification.