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

spec/models/post_stripper_spec.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+RSpec.describe PostStripper do
+  it "strips mentions in quotes" do
+    mention = '<a class="mention">@andrei</a>'
+    cooked = "<aside class='quote'>#{mention}</aside>"
+    fragment = Nokogiri::HTML5.fragment(cooked)
+
+    PostStripper.strip(fragment)
+
+    expect(fragment.to_s).to_not include(mention)
+  end
+
+  it "strips group mentions in quotes" do
+    group_mention = '<a class="mention-group">@moderators</a>'
+    cooked = "<aside class='quote'>#{group_mention}</aside>"
+    fragment = Nokogiri::HTML5.fragment(cooked)
+
+    PostStripper.strip(fragment)
+
+    expect(fragment.to_s).to_not include(group_mention)
+  end
+
+  it "strips oneboxes" do
+    onebox =
+      '<aside class="onebox">
+        Onebox content
+      </aside>'
+    cooked = "<p>#{onebox}</p>"
+    fragment = Nokogiri::HTML5.fragment(cooked)
+
+    PostStripper.strip(fragment)
+
+    expect(fragment.to_s).to_not include(onebox)
+  end
+
+  context "with plugins" do
+    after { DiscoursePluginRegistry.reset_register!(:post_strippers) }
+
+    it "runs strippers registered by plugins" do
+      plugin_element = '<div class="plugin_class"></div>'
+
+      block = Proc.new { |nokogiri_fragment| nokogiri_fragment.css(".plugin_class").remove }
+      plugin = OpenStruct.new(enabled?: true)
+      DiscoursePluginRegistry.register_post_stripper({ block: block }, plugin)
+
+      fragment = Nokogiri::HTML5.fragment("<p>#{plugin_element}</p>")
+
+      PostStripper.strip(fragment)
+
+      expect(fragment.to_s).to_not include(plugin_element)
+    end
+  end
+end