FEATURE: Enforce mention limits for chat messages (#19034)

* FEATURE: Enforce mention limits for chat messages

The first part of these changes adds a new setting called `max_mentions_per_chat_message`, which skips notifications when the message contains too many mentions. It also respects the `max_users_notified_per_group_mention` setting
and skips notifications if expanding a group mention would exceed it.

We also include a new component to display JIT warning for these limits to the user while composing a message.

* Simplify ignoring/muting filter in chat_notifier

* Post-send warnings for unsent warnings

* Improve pluralization

* Address review feedback

* Fix test

* Address second feedback round

* Third round of feedback

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

plugins/chat/config/settings.yml

@@ -104,3 +104,9 @@ chat:
     client: true
     allow_any: false
     refresh: true
+  max_mentions_per_chat_message:
+    client: true
+    type: integer
+    default: 5
+    max: 10
+    min: 0