Clarify project export

[jochenklar]

Rationale / Begründung

Right now, the XML and CSV exports are a bit misleading. It should be made clear that

• only the XML Export can be imported again
• CSV exports are only usefull for to other tools, e.g. Excel

We should also use Content-Disposition: attachment; filename="filename.xml" for the xml files so that they are downloaded immediately by the browser. Maybe with a setting to keep the old behavior for development (I like the other behavior, but it confused people.).

Affected

Users

[jochenklar]

I think we need to add separators to the list, since it can be a bit crowded.