[RulesServer] Ability to fetch subjects in location

[vinayvenu]

In this story, we will add in a SubjectService that will be passed to rules. It should be of the form

{
    getSubjectsInLocation({location, subjectType})
}

Ensure documentation of services is complete.

[vindeolal]

Right now the implementation of this throws a "not supported" error. There were some challenges in implementing it. We need to make an API call from node server to java server for getting the results. This call has to be async and hence we need to make getSubjectsInLocation async, but making getSubjectsInLocation async in the client is difficult, as right now everything is synchronous in the mobile client and we need to change a lot of things to support async behaviour of this method on mobile client.
So moving this card on hold for now.