Add conversations API (#8832)

* Add conversations API

* Add web UI for conversations

* Add test for conversations API

* Add tests for ConversationAccount

* Improve web UI

* Rename ConversationAccount to AccountConversation

* Remove conversations on block and mute

* Change last_status_id to be a denormalization of status_ids

* Add optimistic locking

config/routes.rb

@@ -261,6 +261,7 @@ Rails.application.routes.draw do
       resources :streaming, only: [:index]
       resources :custom_emojis, only: [:index]
       resources :suggestions, only: [:index, :destroy]
+      resources :conversations, only: [:index]
 
       get '/search', to: 'search#index', as: :search