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/requests/chat_controller_spec.rb

@@ -114,7 +114,6 @@ RSpec.describe Chat::ChatController do
             job: Jobs::Chat::ProcessMessage,
             args: {
               chat_message_id: chat_message.id,
-              is_dirty: true,
             },
           ) do
             put "/chat/#{chat_channel.id}/#{chat_message.id}/rebake.json"