Refactor GeoNode API client module and drop legacy support

[giohappy]

The client was created when GeoNode 3 was still around. Today GeoNode 3 is not maintained anymore, and its legacy APIs have been completely replaced by the new ones (v2). The new version of the plugin will remove legacy support and the modules will be restructured and cleaned up, also to make room for the next GeoNode version 5.

• remove support for GeoNode < 4
• rename the module to follow the GeoNode API version naming (v2 at the moment)
• remove the "Ugin API client class" widget from the Connection Configuration widget (see image below)

In the future we will only rely on the API version, but at the moment we don't have an a way to know which API a GeoNode instance supports, except trying using the endpoints.

[Gpetrak]

@giohappy the PR for this issue is available here. Beyond the tasks that are described in the issue above, the following was implemented:

• Merge all the different API clients (regarding to the GeoNode versions) in one class reducing code repetition
• Handling errors related with GeoNode versions >=5 and < 4
• Updating tests

[giohappy]

closed in #288