Remove the `ForkedProjectLink` model
This removes the `ForkedProjectLink` model that has been replaced by the `ForkNetworkMember` and `ForkNetwork` combination. All existing relations have been adjusted to use these new models. The `forked_project_link` table has been dropped. The "Forks" count on the admin dashboard has been updated to count all `ForkNetworkMember` rows and deduct the number of `ForkNetwork` rows. This is because now the "root-project" of a fork network also has a `ForkNetworkMember` row. This count could become inaccurate when the root of a fork network is deleted.