Add Github OAuth

[rafaeldelboni]

• Redirect user to Github OAuth login page and receive token
• Send it to the backend and store in session the generated JWT.
• Use JWT in all subsequent requests
• Replace Login button with Logoff button
• Show controls to add new/edit/remove Notes, new/edit Examples and new/edit/remove See Alsos in Definition Detail View.

Related: https://github.com/clj-codes/docs.backend/issues/14