Improve detection of the GeoNode version

[ricardogsilva]

GeoNode has an unofficial /version.txt endpoint which returns the current GeoNode version. For example:

https://master.demo.geonode.org/version.txt

Lets use it in order to improve our API detection algorithm in order to avoid having the end user perform actions which are not supported by the remote GeoNode.

[ricardogsilva]

#201 already introduces using /version.txt to get the GeoNode version. Now we need to design the various API client classes in order to use those that have correct capabilities for each version