Replace built-in users functionality with the super-admin signup flow

[azasypkin]

Summary

Currently Secutils.dev server relies on the fact that built-in users are either specified through environment variables or CLI arguments. It was supposed to be a temporary workaround to speed-up development and deployment, but it's not something that should be supported in the long run.

Ideally, Secutils.dev should have the super-admin signup flow or something along these lines.

[azasypkin]

Released in https://github.com/secutils-dev/secutils/releases/tag/v1.0.0-beta.1. To sign up as a user with specific subscription (e.g., ultimate) or handle, admin needs to configure user profile via secutils.toml:

[security.preconfigured_users]
"dev@secutils.dev" = { handle = "dev", tier = "ultimate" }

After that, the user can follow the usual signup flow.