let subscriber confirm opt-in only to specific lists if he was added to multiple lists at once

[lukas-mc]

Is your feature request related to a problem? Please describe.
When a new subscriber is created for private lists with double opt in, he receives an opt in mail. When he is added to more than one list this mail will contain all lists he was added by the list-administrator.

He then can only confirm or decline the subscription only for all lists ast once.

Describe the solution you'd like
It would be great if the new subscriber could select the lists hw will confirm the subscription when he selects the ,,confirm" button in the email he got.

When the subscriber selects the ,,confirm" button he is directed to listmonk:9000/subscription/optin

All lists to be confirmed should have the possibility to be selected/unselected on this page.
The subscriber then can decide if he likes to receive mails for all lists or only some selected lists.

With this feature lists administrators are able to add new subscribers to an number of lists at once and the subscriber will not receive to many ,,opt in mails" if he is added to a number of lists.

Regards Lukas