OS4 meeting export

Currently the OS4-meeting export doesn't export the content of the mediafiles, only the meta data of mediafiles will be exported.
Usually there is no need to export the mediafiles, because you could clone it in the OS4-instance.

If someone wants to use such an export as meeting-backup the content of the mediafiles is not included and we should warn the user about this.
Alternatively we could include the mediafiles in the export. But this would lead to big json-files, imagine a video or mega-documents and we have to think about organization-wide resources.
The import technically reads the whole data into the browser and sends it to the backend to store. We may get and currently we have problems to import such big json-files.

Let's look for a good solution!

Start by tracing the OS4-meeting export and import paths, including how mediafile metadata and content move through the browser and backend. Compare warning-only and mediafile-inclusion approaches, then define a solution that handles large files and organization-wide resources without creating impractically large JSON exports.