Add global API_URL for practical use

diskuy/src/CreateThreadForm.js

diskuy/src/CreateTopicForm.js

diskuy/src/Profile/Profile.js

diskuy/src/Threads/Post.js

diskuy/src/Threads/PreviewThread.js

diskuy/src/Threads/Thread.js

diskuy/src/Topic.js

diskuy/src/TopicList.js

diskuy/src/services/api_link.js

diskuy/src/services/auth.service.js

diskuy/src/services/user.service.js