[Is a member of][HBAC rules] 'Add' button functionality

[carma12]

The 'Add' button functionality allows associating a user to a given HBAC rule. The implementation of the addToHbacRules endpoint has been adapted to the fact that multiple entities can be added to a HBAC rule, i.e.: user (hbacrule_add_user), host (hbacrule_add_host), service (hbacrule_add_service), and Source host (hbacrule_add_sourcehost).

This PR depends on this one to be merged: https://github.com/freeipa/freeipa-webui/pull/359