Option to Disable AuthN and AuthZ

**Is your feature request related to a problem? Please describe.**
I run label-studio behind a gateway which already does the AuthN and AuthZ for any request reaching label-studio. For this reason the login screen and user management is unnecessary in my case.

**Describe the solution you'd like**
It would be ideal to have a toggle (e.g. an env variable) to disable AuthN and AuthZ entirely for label-studio. This would ease the use of label-studio in my case, because the users don't have to login.