Allow creating runners in the Web UI (and API)

It would be convenient if a runner could be created in the web UI (and API) and the generated token could then be directly copied to the machine without the need to run gitlab-runner register.

This is useful if the runners are managed with ansible for example. There the config is stored as a template and the token could then be filed in from the inventory where each node gets assigned a token pre-registered in the UI.