Oops, add ActionCable back to the app

Turbo expects this to exist, for features we don't use! I didn't bother
to check if Turbo has a way to turn this off too, I just said fine lol

Turbo worked in development without this because it only loads files as
needed, whereas in production it blocked the app from starting up. But
you can see the error in development by running `rails zeitwerk:check`,
which attempts to preload everything to make sure it all works, and you
get this:

```
NameError: uninitialized constant ActionCable (NameError)

class Turbo::StreamsChannel < ActionCable::Channel::Base
```

Fixed now!

config/application.rb

@@ -9,11 +9,11 @@ require "rails"
 # Disabled:
 # - active_storage/engine
 # - active_job/railtie
-# - action_cable/engine
 # - action_mailbox/engine
 # - action_text/engine
 %w(
   active_record/railtie
+  action_cable/engine
   action_controller/railtie
   action_view/railtie
   action_mailer/railtie