graph: add spinner

When a commit has a large number of dependencies, it can take a while for the server to respond with the JSON. There should be a spinner displayed to show when work is in progress.

Start at the graph view's request path and inspect how it handles the server's JSON response. Add a visible in-progress spinner while that response is pending, and verify it appears for slow dependency loads and disappears when the graph data arrives.

Written by the indexing model from the issue text.