Move thumbnails over to thumbnail pipeline

api/r0/download.go

api/r0/thumbnail.go

database/db.go

database/table_media.go

database/table_thumbnails.go

go.mod

go.sum

pipelines/_steps/datastore_op/put_and_return_stream.go

pipelines/_steps/download/open_stream.go

pipelines/_steps/download/try_download.go

pipelines/_steps/thumbnails/generate.go

pipelines/_steps/thumbnails/pick_dimensions.go

pipelines/pipeline_download/pipeline.go

pipelines/pipeline_thumbnail/pipeline.go

pipelines/pipeline_upload/pipeline.go

pool/init.go

util/cancel_closer.go

util/mime.go

util/time.go