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

app/controllers/api/v1/conversations_controller.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+class Api::V1::ConversationsController < Api::BaseController
+  LIMIT = 20
+
+  before_action -> { doorkeeper_authorize! :read, :'read:statuses' }
+  before_action :require_user!
+  after_action :insert_pagination_headers
+
+  respond_to :json
+
+  def index
+    @conversations = paginated_conversations
+    render json: @conversations, each_serializer: REST::ConversationSerializer
+  end
+
+  private
+
+  def paginated_conversations
+    AccountConversation.where(account: current_account)
+                       .paginate_by_id(limit_param(LIMIT), params_slice(:max_id, :since_id, :min_id))
+  end
+
+  def insert_pagination_headers
+    set_pagination_headers(next_path, prev_path)
+  end
+
+  def next_path
+    if records_continue?
+      api_v1_conversations_url pagination_params(max_id: pagination_max_id)
+    end
+  end
+
+  def prev_path
+    unless @conversations.empty?
+      api_v1_conversations_url pagination_params(min_id: pagination_since_id)
+    end
+  end
+
+  def pagination_max_id
+    @conversations.last.last_status_id
+  end
+
+  def pagination_since_id
+    @conversations.first.last_status_id
+  end
+
+  def records_continue?
+    @conversations.size == limit_param(LIMIT)
+  end
+
+  def pagination_params(core_params)
+    params.slice(:limit).permit(:limit).merge(core_params)
+  end
+end