add login page

[nk-5]

• issue: #25

login page

• add login form, FB login button, sign up button
• implement login form
• login view model
  • auth login for Firestore

[nk-5]

GoogleService-Info.plist はGitHubのrepositoryをprivateにしたらプロジェクト内に追加する https://github.com/nkws/summary/pull/26/commits/a02ce2c44803a7b14e49b11501666299ac4be36b