Move GitLab UI Rails form builder into its own project/gem
In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64260 we added a custom rails form builder for GitLab UI fields. There was [some discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64260#note_623483976) around moving this into its own project and using CI to publish a Ruby gem that can be consumed by `gitlab-org/gitlab`. Two options are: 1. Create a new project 1. Add to the [`gitlab-org/gitlab-ui`](https://gitlab.com/gitlab-org/gitlab-ui) project
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