Enable DirectEdit button on other blobs than the main one

Currently, the nuxeo-drive-server addon only displays the DirectEdit button on the main blob of a document. It would be useful to do add it to other blobs.

The wip-NXDRIVE-1349-direct-edit-attachments branch implements the required changes for this.

What's left to do:
Since it requires improvements on the desktop client as well, we could maybe add a variable that enables/disables this in the server configuration. That way, we can make sure the majority of the users are on a version recent enough to open those links.
We need to add an XML contribution that contains the variable (which we could extend in the future to contain other variables) and define an operation to access the value.