Bridge: improve handling of bundled md

[glpatcern]

Imported from cs3org/wopibridge#9:

Summary: we need to keep CodiMD markdown files as .md and remove the conversion to .zmd. This avoids breaking sharing, versions and CodiMD revisions.

A "cloud-storage-oriented" way to achieve that is to replace the "upload file" dialog in CodiMD with the ownCloud file picker UI being developed. The file picker would need to create a public link on the fly, such that the CodiMD document would directly include a link of the form https://cernbox.cern.ch/index.php/s/<hash>/download (which works).

Another advantage will be that picture links within the .md files won't contain direct links to the CodiMD /uploads path (which would be entirely "disabled"), along with any reverse proxy routing that the service may expose.

This enhancement depends on the availability of the ownCloud/CERNBox file picker UI.

[glpatcern]

As this was actually implemented in CodiMD, we now have both modes supported by the wopiserver: either "embedded CodiMD" in .zmd format, or "plain markdown" with links included by the filepicker.
This was implemented in 2847ccb so this issue can be closed.