QGIS SVIR plugin: put license inside the proper metadata field instead of in the supplemental information

[Paolo Tormene]

Currently, when uploading a layer to from the SVIR plugin to the Platform, the user can select a license from a combobox, which is inserted in the XML that will be read by geonode to fill the metadata, inside the supplemental information field.
We need to change this workflow as follows:
1) the plugin should call an API on the platform, getting the list of available licenses and the links to the corresponding descriptions
2) instead of using the supplemental information field, the license should be inserted into the "license" field of the metadata, which contains the foreign key to the corresponding license (table "licenses"). (Note: table resourcebase contains the whole metadata)

[Paolo Tormene (paolo-tormene)]

Partially addressed by https://github.com/gem/oq-svir-qgis/pull/7