Is your feature request related to a problem? Please describe.

• I tried adding {{ .UnsubscribeURL }} variable to the subscriber-optin-campaign.html template, but I get an error:

Error compiling template: template: base.html:8:66: executing "optin-campaign" at <.UnsubscribeURL>: can't evaluate field UnsubscribeURL in type struct { Lists []models.List; OptinURLAttr template.HTMLAttr }

Describe the solution you'd like

• Right now if someone is imported into Listmonk and sent an Opt-In campaign, the user has the ability to Opt-in / Confirm their subscription
• If they don't want to opt-in, and instead want to be removed from the list (and also avoid being sent multiple Opt-In's in the future) I'd like to provide an "Unsubscribe" or "remove me from this list" link.