Add GET path for finding relationships of a node

[aadarsh-ram]

There is no path for finding relationships (mainly is_child_of relationship) for any node. This must be added to compute the immediate parents and children of a particular node. (Will be helpful for the frontend)

[alexgarel]

You can either do it with an API or add it to the /entry API ? (maybe with a additional_fields=parents) ?

[aadarsh-ram]

You can either do it with an API or add it to the /entry API ? (maybe with a additional_fields=parents) ?

@alexgarel I think having an additional fields property would make the code a little messy on the "database functions" (entries.py) side, with a lot of conditions. It seemed cleaner to make two different paths /entry/{entry}/parents and /entry/{entry}/children instead.