Fixes #1388. Drawback: requires a small schema migration (included).

We need to check with others if we can now apply a schema modification (or we need an official GN 2.0?).
Please @tomkralidis let me know if the link attributes are populated correctly (not sure about extension and mime).
Using the catalog from QGIS MetaSearch now works as expected (the "Add OWS" buttons are enabled when applicable).