Notify when an I-prioritize issue is closed

Author: LeSeulArtichaut

triagebot.toml

@@ -101,6 +101,8 @@ message_on_add = """\
 - Needs `I-nominated`?
 """
 message_on_remove = "Issue #{number}'s prioritization request has been removed."
+message_on_close = "Issue #{number} has been closed while requested for prioritization."
+message_on_reopen = "Issue #{number} has been reopened."
 
 [github-releases]
 format = "rustc"