Provide option to remove fork relationship when forking project

Problem

When forking a repository, users cannot immediately remove the fork relationship when forking. Users must go into project settings to do this later on. Often times, a project will be forked for purposes other than contributing upstream and keeping the fork relationship is unnecessary.

Proposal

Provide a mechanism to remote the fork relationship when forking. a Keep fork relationship option could be checked by default and unchecked when user wants to remove relationship.