Add counter cache && Create node model
Implement node feature.

Finish https://www.apptrajectory.com/toctan/fora/stories/15635159
Modified tests, controller and model

  1. reply model add counter_cache to topic and user
  2. only signed in user can create reply
  3. add test about topics_count, replies_count
Merge branch style code to master

Conflicts:
	app/views/topics/new.html.slim
	spec/acceptance/steps/user_steps.rb
	spec/factories/topics.rb