Automate lint FCP Zulip topic creation with triagebot (#16614)

As discussed in [#clippy > Automatic creation of FCP Zulip
topics](https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/Automatic.20creation.20of.20FCP.20Zulip.20topics/with/575155235),
this PR configures triagebot to automatically create a Zulip topic in
`#clippy` for lint nomination.

It is triggered by the `lint-nominated` label (need to be created).

In addition to containing a link to the pull request, the topic will
also contain a Zulip poll with three per-determined answers: approve,
decline, don't know.

cc @samueltardieu
r? @flip1995

changelog: none

triagebot.toml

@@ -60,6 +60,26 @@ labels = ["S-waiting-on-concerns"]
 [view-all-comments-link]
 threshold = 20
 
+[notify-zulip."lint-nominated"]
+zulip_stream = 577190 # #clippy/fcp
+topic = "FCP rust-clippy#{number}: {title}"
+# Zulip polls may not be preceded by any other text and pings & short links inside
+# the title of a poll don't get recognized. Therefore we need to send two messages.
+message_on_add = [
+    """\
+PR rust-clippy#{number} "{title}" has been nominated for lint inclusion.
+""",
+    """\
+/poll Approve lint inclusion of rust-clippy#{number}?
+approve
+decline
+don't know
+""",
+]
+message_on_remove = "PR rust-clippy#{number} lint nomination has been removed."
+message_on_close = "PR rust-clippy#{number} has been closed. Thanks for participating!"
+message_on_reopen = "PR rust-clippy#{number} has been reopened."
+
 [assign]
 contributing_url = "https://github.com/rust-lang/rust-clippy/blob/master/CONTRIBUTING.md"
 users_on_vacation = [