Enhance JWT checking

[mayrmartin]

As of now only the validity of the JWT is only checked by validating the signature & the expiration time.
This brings up different problems in different domains, i.e.

• Assume a user has an existing & valid token & the admin plans to ban/remove a user. The admin performs the ban/deletion & the API blocks further login-attempts from the user. But the banned user, who still holds a valid token, will still be able to interact with the system as long as the token does not expire / the user does not sign out.
• Assume a user has an existing & valid token. While the session exists, the blockchain infrastructure gets redeployed/reset. When the logged-in user wants to execute blockchain-related procedures (i.e. approve slave node) the call will fail since the address saved in the JWT diverges from the actual address. This scenario will probably only happen in development but can be rated as cross-cutting-concern and could possibly affect other, slightly altered, scenarios that could occur in a production environment.

Both described scenarios could be solved by altering the validation of the JWT. Improvements could contain:

• Alter the existing JWT check function to not only validate signature & expiration time
• an additional reminder to rotate keys when redeploying parts of the system, forcing users to log-in again & therefore get a new JWT.
• Implementation of renew-token: This would allow updating user information stored in the JWT periodically & would avoid forcing the user to re-sign-in once the token expired (probably overkill at this stage)