Add `gitlab_project_issue` resource

Hi :)

It would be great to implement gitlab_project_issue or gitlab_issue resource. The use case is following:

we are using terraform to provision projects from scratch. As media company we've got kind of the same set of issues that are required at the beginnig of the project. We currently manage this manually or with some extra scripts. Would be good to keep terraform as single source of truth and provision standard issues with terraform too.