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/attachment.go

@@ -18,7 +18,6 @@ import (
 )
 
 func renderAttachmentSettings(ctx *context.Context) {
-	ctx.Data["RequireDropzone"] = true
 	ctx.Data["IsAttachmentEnabled"] = setting.AttachmentEnabled
 	ctx.Data["AttachmentAllowedTypes"] = setting.AttachmentAllowedTypes
 	ctx.Data["AttachmentMaxSize"] = setting.AttachmentMaxSize