CodiMD in CERNBox: leveraging the WOPI protocol to provide collaborative markdown editing

<!--HTML-->This contribution describes the integration of CodiMD, a popular markdown nodejs-based editor, in CERNBox. CodiMD is the open-sourced version of a cloud service run by HackMD.io. Designed to store users' content in a relational database and blobs in a filesystem-based or cloud (e.g. S3) storage, there is no provision for interfacing external storages. But thanks to its open REST API, it is straightforward to programmatically pull and push content to it: add a few tweaks and you get a fully fledged collaborative editor integrated in CERNBox. The integration work has been prototyped in the context of the ScienceMesh project, and it is now proposed to CERNBox users. A key aspect lies in the use of the WOPI protocol, which enables storing some arbitrary state within a WOPI lock. Despite locking can only be considered advisory in a sync-and-share context, in this case WOPI locks help designing a stateless integration, where no state is added to the CodiMD application, and a *WOPI Bridge* microservice fills the gap between CodiMD and WOPI.