Reshuffle url previewers to match thumbnailer structure

controllers/preview_controller/preview_controller.go

controllers/preview_controller/preview_resource_handler.go

pipelines/_steps/url_preview/upload_image.go

pipelines/pipeline_preview/pipeline.go

url_previewing/m/errors.go

url_previewing/m/preview_result.go

url_previewing/m/url_payload.go

url_previewing/p/calculated.go

url_previewing/p/oembed.go

url_previewing/p/opengraph.go

url_previewing/u/acl.go

url_previewing/u/http.go

url_previewing/u/summarize.go