FEATURE: Mark all chat channels read with a shortcut (#20629)

This commit adds a keyboard shortcut (Shift+ESC) for chat which marks all
of the chat channels that the user is currently a following member of as read,
updating their `last_read_message_id`. This is done via a new service.

It also includes some refactors and controller changes:

* The old mark message read route from `ChatController` is now supplanted
  by the `Chat::Api::ReadsController#update` route.
* The new controller can handle either marking a single or all messages read,
  and uses the correct service based on the route and params.
* The `UpdateUserLastRead` service is now used (it wasn't before), and has been slightly
  updated to just use the guardian user ID.

plugins/chat/config/routes.rb

@@ -6,6 +6,8 @@ Chat::Engine.routes.draw do
     get "/channels" => "channels#index"
     get "/channels/me" => "current_user_channels#index"
     post "/channels" => "channels#create"
+    put "/channels/read/" => "reads#update_all"
+    put "/channels/:channel_id/read/:message_id" => "reads#update"
     delete "/channels/:channel_id" => "channels#destroy"
     put "/channels/:channel_id" => "channels#update"
     get "/channels/:channel_id" => "channels#show"