tidy up files

frontend/src/App.js

frontend/src/components/GoogleLoginButton.js

frontend/src/components/Header.js

frontend/src/components/Post.js

frontend/src/css/App.css

frontend/src/css/Header.css

frontend/src/css/Search.css

frontend/src/css/index.css

frontend/src/index.js

frontend/src/pages/Login.js

frontend/src/pages/Main.js

frontend/src/pages/PostWrite.js

frontend/src/pages/Search.js