This PR adds instance-level variables, and so closes #27726 ![gitea_instance_variables_1](https://github.com/go-gitea/gitea/assets/8344487/ad409cd4-ce36-4c84-a764-34451b0fb63a) ![gitea_instance_variables_2](https://github.com/go-gitea/gitea/assets/8344487/426f0965-dec6-4560-948c-067cdeddd720) ![gitea_instance_variables_3](https://github.com/go-gitea/gitea/assets/8344487/cf1d7776-4938-4825-922e-cbbbf28a5f33)