models/post.rb: add validate, change after_save to before_save
post_controller.rb: 1. add a action to check whether current user owns the post. 2.add before_action: authenticate_user!
views/posts/_form.html.erb: 1. add option radio_button. 2. add hidden_field, current_user.id.
views/posts/index.html.erb: 1. add notice message. 2. change td post.user -> post.user.username.
Merge pull request #2 from zxlai/feature/link_post_user

Feature/link post user