Subgraph and js

In the branch fix/network, a new subgraph endpoint in included.

• Queries in queries.js need to be modified in order to display:
  - the number of opened channels
  - the number of closed channels
  - address
  - id (multiaddress)
  for each node.
• add function to calculate importance score in api.js
• add function to calculate the integration of importance score over time in api.js

Front end

• getAllAccounts() and getAllData() function need to be adapted to display data in the frontend
• Add another field of importance score

importance(node) = a scalar value that measures the importance of the node to the network.
                                = stake(node) * sum(weight(channel) for all outgoingChannels(node))
                               ```