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/issue.go

@@ -697,7 +697,6 @@ func ViewIssue(ctx *context.Context) {
 	}
 
 	ctx.Data["RequireHighlightJS"] = true
-	ctx.Data["RequireDropzone"] = true
 	ctx.Data["RequireTribute"] = true
 	ctx.Data["RequireSimpleMDE"] = true
 	renderAttachmentSettings(ctx)