feat: notify doers of a merge when automerging (#21553)

I found myself wondering whether a PR I scheduled for automerge was
actually merged. It was, but I didn't receive a mail notification for it
- that makes sense considering I am the doer and usually don't want to
receive such notifications. But ideally I want to receive a notification
when a PR was merged because I scheduled it for automerge.

This PR implements exactly that.

The implementation works, but I wonder if there's a way to avoid passing
the "This PR was automerged" state down so much. I tried solving this
via the database (checking if there's an automerge scheduled for this PR
when sending the notification) but that did not work reliably, probably
because sending the notification happens async and the entry might have
already been deleted. My implementation might be the most
straightforward but maybe not the most elegant.

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

options/locale/locale_en-US.ini

@@ -3003,6 +3003,7 @@ reopen_pull_request = `reopened pull request <a href="%[1]s">%[3]s#%[2]s</a>`
 comment_issue = `commented on issue <a href="%[1]s">%[3]s#%[2]s</a>`
 comment_pull = `commented on pull request <a href="%[1]s">%[3]s#%[2]s</a>`
 merge_pull_request = `merged pull request <a href="%[1]s">%[3]s#%[2]s</a>`
+auto_merge_pull_request = `automatically merged pull request <a href="%[1]s">%[3]s#%[2]s</a>`
 transfer_repo = transferred repository <code>%s</code> to <a href="%s">%s</a>
 push_tag = pushed tag <a href="%[2]s">%[3]s</a> to <a href="%[1]s">%[4]s</a>
 delete_tag = deleted tag %[2]s from <a href="%[1]s">%[3]s</a>