Use Auth0 as authentication service

[eklem]

Auth0 pricing

• Start with Free plan - Up until 7000 users
• Move to Developer plan when able to get paid - Up until 50000 users
• If things go well, change to Enterprise plan when getting closer to 50000 users. It costs more per user and has a starting cost that is fairly high, but still more than manageable.

Other issues concerning authentication:

• Draw the business model - four levels of user engagement
• Levels of user registration
• Create user / login