This will eventually replace the unversioned 'capabilities' call.
Client applications can use this versions call to find out what versions
are available, if they support multiple versions. The capabilities
of each version, and the format of the capabilities call, is then
able to change between versions. If a client only supports one version
they can jump straight to the versioned capabilities call.
Additionally, supported versions are now a list of version identifiers,
without any implications surrounding min/max values (e.g. guesswork
for supported intermediate versions).
Fixes#2162
This will allow us to rename api actions without causing permissions headaches. The choice of
abilities files is made by inheriting from either api_controller or application_controller.
Also rename capabilities to api_capabilites, for consistency.
