Add welcome page to root path
Guest user can see public projects
Align right a link to the function of changing the state of the user
Move import link from home#index to projects#index

Because the link is related to the project
Extract common logic as `visible` scope
Merge pull request #101 from kmiyake/iss-100

#99, #100 トップページの表示を切り替え、ゲストユーザーが公開プロジェクトを確認できる