Add JWT user login

[xsyann]

Allow users to login using a JWT provided in query param.

For instance: https://app.lemverse.com?token={jwt_token} The minimal required payload is the user email in the sub claim:

{
  "sub": "user@example.com"
}

Additional standard claims can be used (for instance to limit the token validity in a time range):

{
  "sub": "user@example.com",
  "nbf": 1667563200,
  "exp": 1667570400,
}

and a custom level_id claim allows to log-in a user in a specific level:

{
  "sub": "user@example.com",
  "level_id": "lvl_nENCytZvbcHhxsFSt"
}

When "jwtAuthSecret" is not provided in the settings, the JWT login handler is completely disabled.

This feature allows to generate time ranged login links from a third party app.