Change 2FA to use hostname as issuer (fixes #4518)

[Nutomic]

Im not sure if this is backwards compatible with existing 2FA tokens or if this is a breaking change.

[dullbananas]

Probably backwards compatible because totp_rs appears to only use the secret when signing timestamps, but I might be wrong

[dessalines]

That sounds right, AFAIK the issuer name is only to help you add it to your TOTP client.

I'm not sure, because it looks like even the check_totp_2fa_valid function requires you to rebuild the TOTP with the issuer name.

@constantoine does a TOTP check require a correct issuer name?

[constantoine]

@dessalines issuer is only useful for generating the URI that will be scanned by the TOTP Client, and is used for disambiguation purposes

The only thing used for a check is indeed the secret key (and timestamp), so this will not affect token generation in any way

[dessalines]

Sweet, thx!