add a domain allow list

Problem/Motivation

ShURLy setting has a check box "Forbid URL target by custom pattern" with a regular expression to define the forbidden urls. My use case is to have a list/expression defining an allowed list (only domains/urls in the allowed list can be created)

Proposed resolution

Ideally we would have an allowed list (text area) with a list of allowed domains, or regex patterns to match against domains. If a long url domain is not on the list the short url ill not be created. Also have a permission to allow allowed role to bypass the domain restrictions.