IOK Picker Dialog UI

[rianadon]

For both selecting an IOK to merge and selecting an iok with the "upload iok" button:

we will need a more capable picker to upload both local IOKs as well as GitHub awesome lists and other IPFS and blockstack IOKs.

This component can have a callback to send the uploaded iok's json. For example, you could initialize a:

<IokPicker
  onPick={json => console.log('chosen iok', json)}
/>

The loadGraph function in SignedIn could be abstracted into a library that isn't tied from the SignedIn component's state,.
The function would need to be extended to handle local files as well as Awesome lists (requires #74), and then it could be used within the picker component.

UI mockup of the picker dialog:

[rustielin]

Maybe integrate the IoK list