Improve explorer visualization to show hermez node synchronization status

[tclemos]

Rationale

When the boot coordinator is under synchronization or it is out of sync, we are not showing this information to the users.

Implementation

Change the UI behavior to show a synchronization state with the information provided by the state API.

As an example, we could use the information lastEthereumBlock and lastSynchedBlock, if they are different, the node is not synchronized.

{
  "network": {
    "lastEthereumBlock": 5463219,
    "lastSynchedBlock": 5463219,
  },
}

Also, the frontend could have another API directly consumed from an Ethereum node in other to have a different source of the lastEthereumBlock.