Respect Form Group ID in Powermail Validators

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The Powermail Validators doesn't respect the Form Group ID, only the ID which is set in the TS.
If you have multiple forms with different Group IDs the Validator doesn't work.

Example:
Opt-Out Form - Group ID: 123456
TS Group ID: 654321

The Opt-Out Validator is using Group ID 654321 and will throw the error that the user is not present in the group, even when he is.

Tried to fix this by myself - but i'm not deep enough into Powermail Validators ;)