User authentication

[erwa]

Currently, Small Board lets anyone create a user and password and see puzzles and submit answers. We should probably have users log in using Google Authentication and have a whitelist of users on Small Board to restrict who can see the puzzles and status.

[erwa]

We can use the list of users who have access to the hunt Google Drive to determine the list of valid users. File metadata has a "permissions" field that contains this info: http://googleapis.github.io/google-api-python-client/docs/dyn/drive_v3.files.html#get

[erwa]

This is what the error message will look like if a user logs in but isn't in the email whitelist:

[erwa]

Added a GOOGLE_DRIVE_HUNT_FOLDER_ID environment variable, which should be set to the id of the base hunt Google Drive folder. The list of whitelisted emails (SOCIAL_AUTH_GOOGLE_OAUTH2_WHITELISTED_EMAILS) will be populated with the emails of the users who have access to the Google Drive folder.

If we add new users to the Google Drive folder, we'll need to restart Small Board for it to refresh the email whitelist.