Add a subgraph (thegraph.com) to access group entries

[glamperd]

Subgraphs should:

• return leaf node details (path, index), given a group and ID commitment
• return all ID commitment leaf nodes for a group

[glamperd]

Initial deployment based on Kovan contract: https://thegraph.com/hosted-service/subgraph/glamperd/interrep-groups-kovan

[stonebell-au]

Merged to develop branch