FIX: user got notified about a mention inside a chat message quote (#24229)

When quoting a chat message in a post, if that message contains a mention, 
that mention should be ignored. But we've been detecting them and sending 
notifications to users. This PR fixes the problem. Since this fix is for 
the chat plugin, I had to introduce a new API for plugins:

    # We strip posts before detecting mentions, oneboxes, attachments etc. 
    # We strip those elements that shouldn't be detected. For example, 
    # a mention inside a quote should be ignored, so we strip it off. 
    # Using this API plugins can register their own post strippers. 
    def register_post_stripper(&block) 
    end

lib/discourse_plugin_registry.rb

@@ -122,6 +122,8 @@ class DiscoursePluginRegistry
 
   define_filtered_register :post_action_notify_user_handlers
 
+  define_filtered_register :post_strippers
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end