show a spinner while the db is loading

[erickt]

it can take a while for the hbase backend to spin up, so while it's loading we should display a spinner until it's ready. To handle the case that we want to show the import dialog on creation, we can add a state flag to the graph in the metagraph. This would let the frontend poll on the graph's load status.

[erickt]

This is partially worked around in #150 and #156, which does synchronous loading of the various graphs. This allows the UI to show the loading bar spinner until the graph is fully created.