[Feature] List sign up link

[Flo-N]

Use case
A link that can be shared to a group chat to allow signing up to a list without the list being publicly visible or needing to embedd the signup form on an external page. This woulb be usefull to manage semi private list, like a newsletter for former club members, it does not need to be super private, having anyone sign up that is visiting the homepage of the club that has other list for non members would not be great either.

Solution proposal
This new sign up link option could be an item like public and privat for the type field when editing a list. The name could be link. The link is generated and shown on the edit page of the list when typ "link" is selected.

Alternatively every list could always have a link that allows sing up that is shown on the edit page of the list.

The link could lead to a page under the listmonk domain that gives an interface like the one already used for signing up to public list.