Manage conflicting import versions

[The4thLaw]

Description

The XML export files have a meta/version/@Schema field that allows identifying the schema with which an export was generated.

We should use this field to warn users when they import a version that is older than the current one. It should still be possible to run the import because chances are that it will work properly.

The warning should show the meta/version/@demyo field rather than the technical schema version.

Maybe we could check .dea files on the client side with a browser zip library (like https://github.com/gildas-lormeau/zip.js). This would really ease the confirmation. This will only work if the zip file is not fully loaded in memory (which seems possible : Stuk/jszip#290)

With that said, since the warning should be quite rare, we could ask the user to upload again if confirmation is needed.