Display items from Index in GUI

[Spedge]

When Hangar is started, the index is updated with the information about what's been stored. Hangar can be stored either in "in-memory" or "zookeeper" mode.

The first task to get the GUI moving is to be able to list all items within the index - either by connecting to zookeeper or by interacting with the in-memory index, depending on Hangar's setup.

We might need a new API to interact with the index if it's "in-memory" but we want to directly interact with the zookeeper cluster if it's in distributed mode - it shouldn't go through the hangar-api instances.