Use sorcery for logins

This gives us more flexibility and allows us to use password authentication
later. Also we don't need to build the login functionality ourself.

Gemfile

@@ -29,6 +29,7 @@ gem 'google-api-client'
 gem 'rack-cors'
 gem 'sentry-raven'
 gem 'sequenced'
+gem 'sorcery'
 
 group :development, :test do
   # Call 'byebug' anywhere in the code to stop execution and get a debugger console