Move dropzone.js to npm/webpack (#10645)

- unvendor dropzone and upgrade it from 4.2.0 to 5.7.0
- make `csrf` available on window.config

routers/repo/editor.go

@@ -474,7 +474,6 @@ func DeleteFilePost(ctx *context.Context, form auth.DeleteRepoFileForm) {
 }
 
 func renderUploadSettings(ctx *context.Context) {
-	ctx.Data["RequireDropzone"] = true
 	ctx.Data["RequireTribute"] = true
 	ctx.Data["RequireSimpleMDE"] = true
 	ctx.Data["UploadAllowedTypes"] = strings.Join(setting.Repository.Upload.AllowedTypes, ",")