Store admin tokens in database

[lus]

Currently, the only possible administration token is stored in the PASTY_MODIFICATION_TOKEN_MASTER environment variable.
I think it would make sense to store Argon2id hashes in an additional admin_tokens database table instead so we can later provide an API to manage them. The current PASTY_MODIFICATION_TOKEN_MASTER value will then be used as the only superadmin token which is authorized to manage other administration tokens.