APIs needed by the app for better user management

[teolemon]

What

API error codes on signup (we're forced to parse an HTML page at the moment)

Beta Give feedback

1. Add an API error code on signup when email already exists
2. Add an API error code on signup when username already exists
3. Add an API error code on signup when name is not empty
4. Add an API error code on signup when password is too short
5. Add an API error code on signup when invalid email

User management APIs

Beta Give feedback

1. Add integrated Login support for the account deletion route
2. Create an API to set the language in which the user wishes to receive the Open Food Facts Newsletter
3. Create an API to unsubscribe from Newsletter
4. Create an API to know whether you're registered to the newsletter
5. Create an API to set/read country language
6. Being able to see the user status

Part of

• 🤳🥫 Support our mobile app (Tracker) #8919