Metadata upload doesn't work for GeoNode 5 instances

[Gpetrak]

The metadata upload doesn't work for GeoNode 5 instances. If a user tried to upload metadata, the plugin presents the message:Could not upload metadata.

Probably this is a bug due to the change of metadata endpoint from GeoNode 4 to GeoNode 5.

[Gpetrak]

Note: A draft PR is created for this issue: #320
However, we have to test it again, (and slightly modify it in case) when we integrate the new version check described in this issue: #318