Implement _get_comm_info function to rehydrate comms

[captainsafia]

We need to implement a _get_comm_info method that requests info about the active comms from the kernel so that we can rehydrate the state when we refresh the page without changing the kernel.

cc: @miduncan

[miduncan]

I looked into this today, and here is my current understanding of how this _get_comm_info flow works and why I don't think it is right for our scenario:

1. _get_comm_info sends a message to the kernel, specifying the 'jupyter.widgets' target
2. Kernel responds by sending a list of comm_ids (which map to model_ids)
3. Manager creates a Comm for each comm_id and sends a request_state message to get the data it needs

In our flow, since we are already given a model_id before we even instantiate Manager, I think we should modify our code to get the model information to be something like this:

try{
  return getModelById(id)
}catch{
  return await requestState(id)
}finally{
  throw ...
}