Implement a way to disable sign up via email

[tess1o]

We need a way how to disable sign up via email For example via env variable: GEOPULSE_EMAIL_SIGNUP_ENABLED=true. If disabled, the sign up API on backend should throw an error. Frontend should show some notification and disable sign up button.

[tess1o]

Implemented in version 1.0.1
The env property value is GEOPULSE_AUTH_SIGN_UP_ENABLED. Default value is true