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

app/models/post_analyzer.rb

@@ -146,11 +146,9 @@ class PostAnalyzer
   def cooked_stripped
     @cooked_stripped ||=
       begin
-        doc = Nokogiri::HTML5.fragment(cook(@raw, topic_id: @topic_id))
-        doc.css(
-          "pre .mention, aside.quote > .title, aside.quote .mention, aside.quote .mention-group, .onebox, .elided",
-        ).remove
-        doc
+        cooked = cook(@raw, topic_id: @topic_id)
+        fragment = Nokogiri::HTML5.fragment(cooked)
+        PostStripper.strip(fragment)
       end
   end
 

app/models/post_stripper.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+# 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.
+class PostStripper
+  def self.strip(nokogiri_fragment)
+    run_core_strippers(nokogiri_fragment)
+    run_plugin_strippers(nokogiri_fragment)
+    nokogiri_fragment
+  end
+
+  private
+
+  def self.run_core_strippers(nokogiri_fragment)
+    nokogiri_fragment.css(
+      "pre .mention, aside.quote > .title, aside.quote .mention, aside.quote .mention-group, .onebox, .elided",
+    ).remove
+  end
+
+  def self.run_plugin_strippers(nokogiri_fragment)
+    DiscoursePluginRegistry.post_strippers.each do |stripper|
+      stripper[:block].call(nokogiri_fragment)
+    end
+  end
+
+  private_class_method :run_core_strippers, :run_plugin_strippers
+end