PERF: cook message in background (#24227)

This commit starts from a simple observation: cooking messages on the hot path can be slow. Especially with a lot of mentions.

To move cooking from the hot path, this commit has made the following changes:

- updating cooked, inserting mentions and notifying user of new mentions has been moved inside the `process_message` job. It happens right after the `Chat::MessageProcessor` run, which is where the cooking happens.
- the similar existing code in `rebake!` has also been moved to rely on the `process_message`job only
- refactored `create_mentions` and `update_mentions` into one single `upsert_mentions` which can be called invariably
- allows services to decide if their job is ran inline or later. It avoids to need to know you have to use `Jobs.run_immediately!` in this case, in tests it will be inline per default
- made various frontend changes to make the chat-channel component lifecycle clearer. we had to handle `did-update @channel` which was super awkward and creating bugs with listeners which the changes of the PR made clear in failing specs
- adds a new `-processed` (and `-not-processed`) class on the chat message, this is made to have a good lifecyle hook in system specs

plugins/chat/spec/system/deleted_message_spec.rb

@@ -15,19 +15,14 @@ RSpec.describe "Deleted message", type: :system do
   end
 
   context "when deleting a message" do
+    fab!(:message_1) { Fabricate(:chat_message, chat_channel: channel_1) }
+
     it "shows as deleted" do
       chat_page.visit_channel(channel_1)
-      channel_page.send_message
 
-      expect(page).to have_css(".-persisted")
+      channel_page.messages.delete(message_1)
 
-      last_message = find(".chat-message-container:last-child")
-      channel_page.messages.delete(OpenStruct.new(id: last_message["data-id"]))
-
-      expect(channel_page.messages).to have_deleted_message(
-        OpenStruct.new(id: last_message["data-id"]),
-        count: 1,
-      )
+      expect(channel_page.messages).to have_deleted_message(message_1, count: 1)
     end
 
     it "does not error when coming back to the channel from another channel" do